Interactions are tricky. The Koinup photo website doesn't seem to allow hot linking, so here's a cold link to what can happen currently when two KittyCatS collide:
http://www.koinup.com/LGwuzhere/work/441928/
Hm. If moving interactions are too tricky or will take a long time to develop, how about starting with one in which a cat can detect and lie down next to another close enough as if sharing warmth? That may not be *too* different from the coding of how they already interact with the Scratch-n-Play Post. The find- and- lie- next- to action itself would only be performed by one cat.