I've only had about three litters so far but both moms were first time moms. I didn't have any issues with them being housed together. One gave birth about five days before the other. Once they both had litters I noticed after a few days there would be some of eachs in each nest. When the babies got older the nests were completely mixed of whos was whos. Never saw any fighting over babies. They just picked up the strays and returned it to their own nest, and nursed each others babies.