[Hamtastic]

2 of each thing is not going to guarantee lack of falling out. It is a general guideline that should work for many pairs, but what works for some, might not work for some others, and some experimenting through trial and error might be necessary. And then there are pairs who are just not meant to be no matter how many guidelines are followed, and some that do best with a single wheel, a single house, and a smaller home. My friend has two very tightly knit robo pairs who each share one wheel, and I believe it works for them (and I think Erin's had a similar experience with her colony). Those guidelines are just to minimize the chances of falling out, not to prevent them.