I keep two females together to birth alot. They help each other out raising the babies. Sometimes, they will drag the babies from one nest to the other for hours at a time, can be humourous at times...

Very rarely do I have females that kill each others babies, but it does happen from time to time.

I also will take huge litters of 15 or more, and seperate a few of them into a mom that may only have 5 or 6, kind of equal out the litter sizes without problems.
Rats are very good mothers to theirs and other pups...