How can I combining two players to make one ?

There are lots of ways to go about what you're asking. What I'd do (in point form) is:

• in the ship collider check if I'm colliding with another ship

• if it's another ship, child the shooter one to the steering one (up to you how you differentiate them). The transform of the child will be relative to the parent.

• for the shooter shit set a bool to indicate it no longer can steer, in the onupdate portion that checks direction input

  bool canSteer = true; //in initial object

  canSteer = false; //once attached

     //in input on update code
       if(input.GetKey(player2left)&& canSteer)
       {
          //only do steering if enabled
       }
  
  

• for the driver/parent do something similar to disable shooting.

I'd probably do a single script for both types of ship (so I can reuse it on both) and just set the ship type via an enum.

Something like the following (note: this is an example to get you started, don't expect it to work.. I mean it compiles, but that's to make sure the syntax is ok):

 using UnityEngine;
 using System.Collections;
 
 public class CombiningShip : MonoBehaviour {
     public enum ShipType { shooter, steerer};
     public ShipType shipType;
     
     public enum ControlInputs { player1, player2};
     public ControlInputs inputToUse;
     
     public bool CanSteer;
     public bool CanFire;
     
     string player1firekey = "b";//just for example so code compiles, not the way to do this
     string player1moveleft = "a";//again for example
     
     
     void Start () 
     {
         CanFire = true;
         CanFire = true;
     
     }
     
     
     void Update () 
     {
         switch (inputToUse)  //for example, plenty of ways to do this
         {
         case ControlInputs.player1: 
         {
             if( Input.GetKey(player1firekey) && CanFire )
             {
                 //do fire    
             }
             if(Input.GetKey (player1moveleft) && CanSteer)
             {
                 //move left    
             }
             
             
         }
             break;
         case ControlInputs.player2:
             //do stuff for 2nd play controls
             break;
         
         }
     
         
     }
     
     
     void OnTriggerEnter(Collider other)
     {        
         var possibleShip = other.GetComponent<CombiningShip>();
         if(possibleShip != null)
         {
             if(possibleShip.shipType==ShipType.shooter)//other ship is shooter
             {
                 possibleShip.CanSteer = false; //stop if from being able to shoot
                 possibleShip.transform.parent = this.transform; //child ship to this one
                 this.CanFire = false; //stop this one from firing
             }
         }
     }
 }