Changing 2 dependent variable at the same time

Here is an example of a engine script found on the wiki (http://wiki.unity3d.com/index.php?title=JCar) :

 /**
  * A simple car physics script using wheel colliders.
  * Jaap Kreijkamp [jaap] at [ctrl-j.com.au]
  *
  * orientation should be that front of car is in direction of
  * the 'blue arrow' in Unity, the roof should be in direction of
  * the green angle. Thus with rotation 0, 0, 0, adding 1 to Z
  * will move car 1m forward, adding 1 to Y will move car 1m
  * upward. The wheels should be children of the car object this
  * script is added to and connected to the wheelFL, wheelFR, ...
  * variables.
  *
  * Please modify script and do whatever you like with it,
  * in it's current state it should give a working car,
  * but by no means perfect (or even close) behavior.
  * It's my first attempt and don't really need in my current
  * project so haven't put too much effort into it to perfect
  * it. As often people are looking for help to getting
  * a car working with wheel colliders, I'd appreciate when
  * improvements are posted back on the unity forums.
  *
  * Lastly, thanks to all the people helping me on forums (especially
  * the order of initialisation problem and other example
  * code that helped me much in learning how to do stuff like
  * this).
  */
  
 using UnityEngine;
 using System.Collections;
  
 public enum JWheelDrive {
     Front = 0,
     Back = 1,
     All = 2
 }
  
  
 public class JCar : MonoBehaviour { 
  
     // if connected the controls will block if object not  active
     // (for example steer only if car camera is active).
     public GameObject checkForActive;
  
     public Transform wheelFR; // connect to Front Right Wheel transform
     public Transform wheelFL; // connect to Front Left Wheel transform
     public Transform wheelBR; // connect to Back Right Wheel transform
     public Transform wheelBL; // connect to Back Left Wheel transform
  
     public float suspensionDistance = 0.2f; // amount of movement in suspension
     public float springs = 1000.0f; // suspension springs
     public float dampers = 2f; // how much damping the suspension has
     public float wheelRadius = 0.25f; // the radius of the wheels
     public float torque = 100f; // the base power of the engine (per wheel, and before gears)
     public float brakeTorque = 2000f; // the power of the braks (per wheel)
     public float wheelWeight = 3f; // the weight of a wheel
     public Vector3 shiftCentre = new Vector3(0.0f, -0.25f, 0.0f); // offset of centre of mass
  
     public float maxSteerAngle = 30.0f; // max angle of steering wheels
     public JWheelDrive wheelDrive = JWheelDrive.Front; // which wheels are powered
  
     public float shiftDownRPM = 1500.0f; // rpm script will shift gear down
     public float shiftUpRPM = 2500.0f; // rpm script will shift gear up
     public float idleRPM = 500.0f; // idle rpm
  
     public float fwdStiffness = 0.1f; // for wheels, determines slip
     public float swyStiffness = 0.1f; // for wheels, determines slip
  
     // gear ratios (index 0 is reverse)
     public float[] gears = { -10f, 9f, 6f, 4.5f, 3f, 2.5f };
  
     // automatic, if true car shifts automatically up/down
     public bool automatic = true;
  
     public float killEngineSoundTimeout = 3.0f; // time until engine sound is cut off (in s.)
  
     // table of efficiency at certain RPM, in tableStep RPM increases, 1.0f is 100% efficient
     // at the given RPM, current table has 100% at around 2000RPM
     float[] efficiencyTable = { 0.6f, 0.65f, 0.7f, 0.75f, 0.8f, 0.85f, 0.9f, 1.0f, 1.0f, 0.95f, 0.80f, 0.70f, 0.60f, 0.5f, 0.45f, 0.40f, 0.36f, 0.33f, 0.30f, 0.20f, 0.10f, 0.05f };
  
     // the scale of the indices in table, so with 250f, 750RPM translates to efficiencyTable[3].
     float efficiencyTableStep = 250.0f;
  
     int currentGear = 1; // duh.
  
     // shortcut to the component audiosource (engine sound).
     AudioSource audioSource;
  
     // every wheel has a wheeldata struct, contains useful wheel specific info
     class WheelData {
         public Transform transform;
         public GameObject go;
         public WheelCollider col;
         public Vector3 startPos;
         public float rotation = 0.0f;
         public float maxSteer;
         public bool motor;
     };
  
     WheelData[] wheels; // array with the wheel data
  
     // setup wheelcollider for given wheel data
     // wheel is the transform of the wheel
     // maxSteer is the angle in degrees the wheel can steer (0f for no steering)
     // motor if wheel is driven by engine or not
     WheelData SetWheelParams(Transform wheel, float maxSteer, bool motor) {
         if (wheel == null) {
             throw new System.Exception("wheel not connected to script!");
         }
         WheelData result = new WheelData(); // the container of wheel specific data
  
         // we create a new gameobject for the collider and move, transform it to match
         // the position of the wheel it represents. This allows us to do transforms
         // on the wheel itself without disturbing the collider.
         GameObject go = new GameObject("WheelCollider");
         go.transform.parent = transform; // the car, not the wheel is parent
         go.transform.position = wheel.position; // match wheel pos
  
         // create the actual wheel collider in the collider game object
         WheelCollider col = (WheelCollider) go.AddComponent(typeof(WheelCollider));
         col.motorTorque = 0.0f;
  
         // store some useful references in the wheeldata object
         result.transform = wheel; // access to wheel transform 
         result.go = go; // store the collider game object
         result.col = col; // store the collider self
         result.startPos = go.transform.localPosition; // store the current local pos of wheel
         result.maxSteer = maxSteer; // store the max steering angle allowed for wheel
         result.motor = motor; // store if wheel is connected to engine
  
         return result; // return the WheelData
     }
  
     // Use this for initialization
     void Start () {
         // 4 wheels, if needed different size just modify and modify
         // the wheels[...] block below.
         wheels = new WheelData[4];
  
         // setup wheels
         bool frontDrive = (wheelDrive == JWheelDrive.Front) || (wheelDrive == JWheelDrive.All);
         bool backDrive = (wheelDrive == JWheelDrive.Back) || (wheelDrive == JWheelDrive.All);
  
         // we use 4 wheels, but you can change that easily if neccesary.
         // this is the only place that refers directly to wheelFL, ...
         // so when adding wheels, you need to add the public transforms,
         // adjust the array size, and add the wheels initialisation here.
         wheels[0] = SetWheelParams(wheelFR, maxSteerAngle, frontDrive);
         wheels[1] = SetWheelParams(wheelFL, maxSteerAngle, frontDrive);
         wheels[2] = SetWheelParams(wheelBR, 0.0f, backDrive);
         wheels[3] = SetWheelParams(wheelBL, 0.0f, backDrive);
  
         // found out the hard way: some parameters must be set AFTER all wheel colliders
         // are created, like wheel mass, otherwise your car will act funny and will
         // flip over all the time.
         foreach (WheelData w in wheels) {
             WheelCollider col = w.col;
             col.suspensionDistance = suspensionDistance;
             JointSpring js = col.suspensionSpring;
             js.spring = springs;
             js.damper = dampers;            
             col.suspensionSpring = js;
             col.radius = wheelRadius;
             col.mass = wheelWeight;
  
             // see docs, haven't really managed to get this work
             // like i would but just try out a fiddle with it.
             WheelFrictionCurve fc = col.forwardFriction;
             fc.asymptoteValue = 5000.0f;
             fc.extremumSlip = 2.0f;
             fc.asymptoteSlip = 20.0f;
             fc.stiffness = fwdStiffness;
             col.forwardFriction = fc;
             fc = col.sidewaysFriction;
             fc.asymptoteValue = 7500.0f;
             fc.asymptoteSlip = 2.0f;
             fc.stiffness = swyStiffness;
             col.sidewaysFriction = fc;
         }
  
         // we move the centre of mass (somewhere below the centre works best.)
         rigidbody.centerOfMass += shiftCentre;
  
         // shortcut to audioSource should be engine sound, if null then no engine sound.
         audioSource = (AudioSource) GetComponent(typeof(AudioSource));
         if (audioSource == null) {
             Debug.Log("No audio source, add one to the car with looping engine noise (but can be turned off");
         }
  
     }
  
     void Update() {
         if (Input.GetKeyDown("page up")) {
             ShiftUp();
         }
         if (Input.GetKeyDown("page down")) {
             ShiftDown();
         }
     }
  
     float shiftDelay = 0.0f;
  
     // handle shifting a gear up
     public void ShiftUp() {
         float now = Time.timeSinceLevelLoad;
  
         // check if we have waited long enough to shift
         if (now < shiftDelay) return;
  
         // check if we can shift up
         if (currentGear < gears.Length - 1) {
             currentGear ++;
  
             // we delay the next shift with 1s. (sorry, hardcoded)
             shiftDelay = now + 1.0f;
         }
     }
  
     // handle shifting a gear down
     public void ShiftDown() {
         float now = Time.timeSinceLevelLoad;
  
         // check if we have waited long enough to shift
         if (now < shiftDelay) return;
  
         // check if we can shift down (note gear 0 is reverse)
         if (currentGear > 0) {
             currentGear --;
  
             // we delay the next shift with 1/10s. (sorry, hardcoded)
             shiftDelay = now + 0.1f;
         }
     }
  
     float wantedRPM = 0.0f; // rpm the engine tries to reach
     float motorRPM = 0.0f;
     float killEngine = 0.0f;
  
     // handle the physics of the engine
     void FixedUpdate () {
         float delta = Time.fixedDeltaTime;
  
         float steer = 0; // steering -1.0 .. 1.0
         float accel = 0; // accelerating -1.0 .. 1.0
         bool brake = false; // braking (true is brake)
  
         if ((checkForActive == null) || checkForActive.active) {
             // we only look at input when the object we monitor is
             // active (or we aren't monitoring an object).
             steer = Input.GetAxis("Horizontal");
             accel = Input.GetAxis("Vertical");
             brake = Input.GetButton("Jump");
         }
  
         // handle automatic shifting
         if (automatic && (currentGear == 1) && (accel < 0.0f)) {
             ShiftDown(); // reverse
         }
         else if (automatic && (currentGear == 0) && (accel > 0.0f)) {
             ShiftUp(); // go from reverse to first gear
         }
         else if (automatic && (motorRPM > shiftUpRPM) && (accel > 0.0f)) {
             ShiftUp(); // shift up
         }
         else if (automatic && (motorRPM < shiftDownRPM) && (currentGear > 1)) {
             ShiftDown(); // shift down
         }
         if (automatic && (currentGear == 0)) {
             accel = - accel; // in automatic mode we need to hold arrow down for reverse
         }
         if (accel < 0.0f) {
             // if we try to decelerate we brake.
             brake = true;
             accel = 0.0f;
             wantedRPM = 0.0f;
         }
  
         // the RPM we try to achieve.
         wantedRPM = (5500.0f * accel) * 0.1f + wantedRPM * 0.9f;
  
         float rpm = 0.0f;
         int motorizedWheels = 0;
         bool floorContact = false;
  
         // calc rpm from current wheel speed and do some updating
         foreach (WheelData w in wheels) {
             WheelHit hit;
             WheelCollider col = w.col;
  
             // only calculate rpm on wheels that are connected to engine
             if (w.motor) {
                 rpm += col.rpm;
                 motorizedWheels++;
             }
  
             // calculate the local rotation of the wheels from the delta time and rpm
             // then set the local rotation accordingly (also adjust for steering)
             w.rotation = Mathf.Repeat(w.rotation + delta * col.rpm * 360.0f / 60.0f, 360.0f);
             w.transform.localRotation = Quaternion.Euler(w.rotation, col.steerAngle, 0.0f);
  
             // let the wheels contact the ground, if no groundhit extend max suspension distance
             Vector3 lp = w.transform.localPosition;
             if (col.GetGroundHit(out hit)) {
                 lp.y -= Vector3.Dot(w.transform.position - hit.point, transform.up) - col.radius;
                 floorContact = floorContact || (w.motor);
             }
             else {
                 lp.y = w.startPos.y - suspensionDistance;
             }
             w.transform.localPosition = lp;
         }
         // calculate the actual motor rpm from the wheels connected to the engine
         // note we haven't corrected for gear yet.
         if (motorizedWheels > 1) {
             rpm = rpm / motorizedWheels;
         }
  
         // we do some delay of the change (should take delta instead of just 95% of
         // previous rpm, and also adjust or gears.
         motorRPM = 0.95f * motorRPM + 0.05f * Mathf.Abs(rpm * gears[currentGear]);
         if (motorRPM > 5500.0f) motorRPM = 5500.0f;
  
         // calculate the 'efficiency' (low or high rpm have lower efficiency then the
         // ideal efficiency, say 2000RPM, see table
         int index = (int) (motorRPM / efficiencyTableStep);
         if (index >= efficiencyTable.Length) index = efficiencyTable.Length - 1;
         if (index < 0) index = 0;
  
         // calculate torque using gears and efficiency table
         float newTorque = torque * gears[currentGear] * efficiencyTable[index];
  
         // go set torque to the wheels
         foreach (WheelData w in wheels) {
             WheelCollider col = w.col;
  
             // of course, only the wheels connected to the engine can get engine torque
             if (w.motor) {
                 // only set torque if wheel goes slower than the expected speed
                 if (Mathf.Abs(col.rpm) > Mathf.Abs(wantedRPM)) {
                     // wheel goes too fast, set torque to 0
                     col.motorTorque = 0;
                 }
                 else {
                     // 
                     float curTorque = col.motorTorque;
                     col.motorTorque = curTorque * 0.9f + newTorque * 0.1f;
                 }
             }
             // check if we have to brake
             col.brakeTorque = (brake)?brakeTorque:0.0f;
  
             // set steering angle
             col.steerAngle = steer * w.maxSteer;
         }
  
         // if we have an audiosource (motorsound) adjust pitch using rpm        
         if (audioSource != null) {
             // calculate pitch (keep it within reasonable bounds)
             float pitch = Mathf.Clamp(1.0f + ((motorRPM - idleRPM) / (shiftUpRPM - idleRPM) * 2.5f), 1.0f, 10.0f);
             audioSource.pitch = pitch;
  
             if (motorRPM > 100) {
                 // turn on sound if it's not playing yet and RPM is > 100.
                 if (!audioSource.isPlaying) {
                     audioSource.Play();
                 }
                 // how long we should wait with engine RPM <= 100 before killing engine sound
                 killEngine = Time.time + killEngineSoundTimeout;
             }
             else if ((audioSource.isPlaying) && (Time.time > killEngine)) {
                 // standing still, kill engine sound.
                 audioSource.Stop();
             }
         }
     }
  
     public void OnGUI() {
         if (checkForActive.active) {
             // calculate actual speed in Km/H (SI metrics rule, so no inch, yard, foot,
             // stone, or other stupid length measure!)
             float speed = rigidbody.velocity.magnitude * 3.6f;
  
             // message to display
             string msg = "Speed " + speed.ToString("f0") + "Km/H, " + motorRPM.ToString("f0") + "RPM, gear " + currentGear; //  + " torque " + newTorque.ToString("f2") + ", efficiency " + table[index].ToString("f2");
  
             GUILayout.BeginArea(new Rect(Screen.width -250 - 32, 32, 250, 40), GUI.skin.window);
             GUILayout.Label(msg);
             GUILayout.EndArea();
         }
     }
 }

I don't know quite what you're wanting to do... You don't need to simultaneously update 2 variables for the engine script to work. You can do it in order.

Another great resource is: http://www.asawicki.info/Mirror/Car%20Physics%20for%20Games/Car%20Physics%20for%20Games.html which contains a lot of math formulas and information.