I had some girl buns together, there were two and later on... three.
I have had a few unintended litters with these girls, and, because it was accidental, generally, it was only one or two that got pregnant.
The first litter that happened was when I had Tabby and Tipsy together... Tabby had babies the one night and Tipsy the night after. They stayed together and all the kits snuggled together in a big group.
When I had the three girls together, two of them got pregnant, again I did not seporate them and there were no problems.
None of these girls were sisters and obv werent spayed because that was a while ago and it was never suggested or thought of.
Thus... I have never seporated my does and never had ANY issues.
But its your choice... I guess it depends how well the does get on with each other and if theyre both ok with kits.... the one of my does, Tabby, loved kits, and tried rearing the other buns kits... bless. She was Aunty Tabby.
Anyways... good luck