Collision Detection dilemma

I’ve been researching collison detection so I can apply it to my project but I am getting totally LOST in all the complexities!

If you have 2 button movie clips on the stage and, when you click either of them, they move to a specific xy location, how do you apply collison detection so that if the space is occupied by either one of them, the other will have to go back to it’s original position until the space becomes available?

The registration point is centered in both movie clips.

If anyone can help me, I’d be very grateful!