Why isn't this input working?

I tried putting two keys doing the same action, but it gives me an

 Assets/Scripts/BallControl.js(15,29): BCE0077: It is not possible to invoke an expression of type 'boolean'

I'm fairly new to making games and coding ^^ all help would make my day

 #pragma strict
 
 var rotationSpeed = 200;
 var jumpHeight = 9;
 
 private var isFalling = false;
 
 function Update ()
 {
     
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     
     if (Input.GetKeyDown(KeyCode.Space)(Input.GetKeyDown(KeyCode.UpArrow) && isFalling == false))
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
 }
 
 function OnCollisionStay ()
 {
     isFalling = false;
 }