You can double the number of females in rotation. I always have the same number of females in the breeder bins.
I've found that I lose more than half the babies if I have more than 2 females in one bin. One female seems to end up doing all the work and see can't handle the work load so the babies suffer. Another good point for birthing bins is they allow you to see if a mother is doing her job and is a good mom or eating babies or just not having that many babies per litter.