I recently tried the birthing together thing and wasn't thrilled with the results so I've gone back to the one mom per 10 gallon way I've always done it. What I am doing is letting them stay in that 10 gallon until the litter is a good couple of weeks old. Making sure I can see they are up and around and healthy and so forth and momma rat's in control of the horde. Then I'll sometimes put two momma's that get along in a huge tub with both their litters of similar ages. That seems to give the little rats some extra playmates and the mom's share the duties of managing them. If I need to remember a specific offspring to hold back or so forth I mark there tail with a bright red marker if I can't visually remember who is who. I exclude from this shared parenting any female rats who have litters I'm tracking specifically for any potential of genetic issues or concern about the mother's ability to care for her litter.