I do keep mine together, but I wouldn't advise anyone else to do it. Mine are in a permanent run, which is 9'x6' and have lots of room. The bunnies will sit on top of the hutches, and the piggies in their nesting boxes if they want to stay out of the way. I must say that quite often, they all sit sunbathing together, and there are a fair few tussles over the best leaf of greens - the piggies usually win :shock: :lol:
However, I think generally they're better off separate.