Attaching multiple rigidbodies together and spawnning as prefabs

I need a way in game to take two rigidodies in the world and fuse them together, i need them to work as one object, so parenting wont work. After than i take that object and i save it to a prefab i have. then i can freely spawn that prefab in the world. The spawning works and saving to prefab works, i just need a way of fusing to objects to eachother and act as if they were one(both moving relative to the other and shareing colliders.)

heres my code:

 var changeto: GameObject;
 
 function Update () {
 
 
     if(Input.GetMouseButtonDown(0))
     {
        var INHAND : GameObject;
        INHAND = Instantiate(AssetDatabase.LoadAssetAtPath("Assets/ball.prefab",GameObject),transform.position,transform.rotation) as GameObject;
        INHAND.rigidbody.velocity = transform.TransformDirection (Vector3.forward * 2);
     }
 
     if(Input.GetMouseButtonDown(1))
     {
        var hit : RaycastHit;
         if(Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), hit))
         {
             Debug.Log(hit.collider.name);
            changeto=hit.transform.gameObject;
         }
         PrefabUtility.ReplacePrefab(changeto,AssetDatabase.LoadAssetAtPath("Assets/ball.prefab",Rigidbody), ReplacePrefabOptions.ConnectToPrefab);        
     }
 }