I wait at least 4-5 days before putting moms together. I've found that until the babies have fur, any stress risks mom killing the babies. But when you do put them together, start with a clean cage that doesn't smell like either of them, put each group of babies in the two farthest corners from each other, and put the moms in at the same time. I'd watch them close for the first 30 minutes or until they settle down with the kids. If they're good moms, eventually they'll probably just pile everybody into one group and take turns laying with them.
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