How to fire two bullets instead of one?

I'm not 100% sure how your code works but you can fire twice or x-amount of times using for-loop.

 if (hasMachineGun) {
     GameObject cloneMachineGun;
     Vector3 fireMachineGun = transform.forward * projectileSpeed;
     
     if(Input.GetButtonDown("Fire1")){
         for (int i = 0; i < 2; i++){    // Fire twice.
             cloneMachineGun = (GameObject)Instantiate(machineGun, machineGunSocket.transform.position, transform.rotation);
             cloneMachineGun.rigidbody.AddForce(fireMachineGun);
         }
         
         playerState = PlayerState.CarNormal;
         changeState = true;
     }
 }

Adding delay is a little more complicated thing. You could use invoke or coroutine. In both cases you have to make function for firing. I made a coroutine example for you, but you should definitely check how they work: http://unity3d.com/learn/tutorials/modules/intermediate/scripting/coroutines

     if (hasMachineGun) {
 
         
         if(Input.GetButtonDown("Fire1")){
 
             StartCoroutine(FireMachineGun());
 
             playerState = PlayerState.CarNormal;
             changeState = true;
         }
     }
 
 float fireDelay = 0.2f; // Delay before both shots
 
 IEnumerator FireMachineGun()
 {
     GameObject cloneMachineGun;
     Vector3 fireMachineGun = transform.forward * projectileSpeed;
 
     for (int i = 0; i < 2; i++){
         yield return new WaitForSeconds(fireDelay);
 
         cloneMachineGun = (GameObject)Instantiate(machineGun, machineGunSocket.transform.position, transform.rotation);
         cloneMachineGun.rigidbody.AddForce(fireMachineGun);
     }
 }