You shouldn't have any problems keeping mothers together. If they both have litters at the same time (not necessarily the same age) you find that they take turns feeding each others babies and you'll find them switching babies back and forth with no problems. I used to switch them into a nursing tank but now I just leave them alone and pull the babies once they are weaned and move them to a grow up tank. One of mine just had 11 babies a couple nights ago and another in the same cage is ready to drop any day now so I'll be watching the switching process soon. Good luck and enjoy your free rats (not free but you know what I mean), Scott.
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Injustice anywhere is a threat to justice everywhere.
Martin Luther King, Jr.
"In any civilized society, it is every citizen's responsibility to obey just laws.
But at the same time, it is every citizen's responsibility to disobey unjust laws."
—Martin Luther King Jr