Snaping/Merging game objects together

I have two types of game objects "redBall" and "blackBall".

They are moving in z direction at random speeds.

What would be a good way of writing a script so that if they collide they would move forwards as one (with black balls speed). I want it to almost look like they are one object instead of two.

On my blackBall I have a script that has this. (Also, blackBall is a child of an empty object)

 void OnCollisionEnter(Collision col) {
     if (col.gameObject.name == "RedBall") {
         Debug.Log ("Collision");
         col.gameObject.transform.parent = transform;            
     }
 }

With that last line i am trying to merge both objects so they have same parent hoping they would behave as one object.

I think i could accomplish what i want if I modify the speed of one object to match the speed of the other object if collision occurs but i feel like there might be a better solution to this.