Years ago we had two pregnant strays show up at the same time. They absolutely insisted on having their kittens in the same box together even though we made separate nests for them. I thought the second mom to go into labor would kill me when i tried to make her stay in the basement to have her litter.
Eventually the two Moms were happy as clams in the same box, sharing nursing and cleaning duties. i honestly couldn't say which kittens were birth by which Mom. This picture reminds me so much of that happy occasion.