Go from mouse to multitouch?

Hey I am making a game were you are controlling 2 turrets. I have asigned a script to both of the turrets that makes them follow the mouse like this:

This is the script:

 #pragma strict
 
 function Start () {
 
 }
 
  var mouse_pos : Vector3;
  var target : Transform; //Assign to the object you want to rotate
  var object_pos : Vector3;
  var angle : float=180;
  
  function Update ()
  {
      mouse_pos = Input.mousePosition;
      mouse_pos.z = 5.23; //The distance between the camera and object
      object_pos = Camera.main.WorldToScreenPoint(target.position);
      mouse_pos.x = mouse_pos.x - object_pos.x;
      mouse_pos.y = mouse_pos.y - object_pos.y;
      angle = Mathf.Atan2(mouse_pos.y, mouse_pos.x) * Mathf.Rad2Deg;
      transform.rotation = Quaternion.Euler(Vector3(0, 0, angle));
      };

What i want to do is making them follow your touch seperatly like this:

Imagine the screen is split in half were the left side controlls the movement of the left turret and the right side controls the right turret.

They only move when you touch the screen, when you move your fingers away they should stay in the same position as they where left. I would also like to implement an angle intervall were the turrets can move between, so that the pipe dont touch their "stand", maybe between 20 and 160 degrees.

I realize that these things are not easy tasks, but can you try to give me some input on how and where to start? Any thought on how to do all this?