and we are looking for a specific tool that firmly holds the balls in place so we can slap them and pull them at the same time, something with a rope.  any suggestions?

Look into a Humbler although there usually isn't rope involved.

I especially like to separate them when I tie them, and I love to tie them so tight they get shiny

cock rings, dental floss, your hands, or feet, rope, but I love utilizing house hold items cooking string works well.

It it wasn't as good as the suggestions!  Thank you and happy slapping!