If they are from the same place and have been kept in the same cage, then I would say they would be fine together, but just until they have babies. When they are getting ready to have them, into a separate tank they go. We kept our female rat in a separate cage for about 2 weeks of her pregnancy, and her babies are now 4 days old and adorable. Plus, I would think it to be alot less stressful not having another grumpy female in there. At school, we have 3 female mice that were kept together, then one had babies, and the other 2 ate the majority of the babies, except for 2. Then mom ended up not caring for them, so my friend took them home and hand raised one of them(the other died).