I have a tip, place the 2 wheels close to one another. I placed mine on each end of the cage and the robo's were fighting over one. I don't know why they did that, I think that one robo was too lazy to walk to the other wheel. but they are alright now. They happily swap wheels when one decides to push one off lol.
I absolutely love the zz2, territorial behaviours don't seem to be an issue with mine