I love it when ratties make their own choices. I initially thought each of these boys would be living alone. Colossus was always kind of a loner. However, I did allow them an opportunity for supervised visits by leaving the doors open to the cages and watching what happened. I would close the cages when not there, but as I watched, over about three days, Colossus moved in with Metoo. Both are males, but Metoo is a friendly chap (he outlived his cage mate). Colossus surprised me because he's always been such a longer -- I rescued him as a lone rat and he has shown little interest in moving in with others. But, they decided they were compatible housemates and now they are in the same house. It's really cute. No fights, no issues.