FaunaClassifieds - Mice Breeders, need some advice!

I have two females that recently gave birth. One the night before last (black), and the other last night (brown). Both decided to nest in the same box, and both were laying with the black one's litter before the brown one gave birth. There are 18 babies in total.

Now, though, I haven't seen the brown female in the box since she gave birth. The black one has both litters, and is being a very good mom, but I am worried that tending to both litters may be too much for just her.

Is there anything I can do to help her? I have another female that had a litter a few weeks ago (the litter was given to a friend for his snakes), could introducing her possibly help? She was a good mom to her litter.

Only reason I left these two girls together was because I was hoping they would both care for both litters, like some mice will. Both became pregnant at the same time, so I knew they'd birth around the same time. Not sure what to do with the brown mouse. Hoping she'll start tending to the litters with the black one soon.

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