We had two unspayed female cats when I was in my early teens. They both became pregnant, one a couple weeks before the other. We prepared a box with comfortable sheets for the first to have her kittens in, which she did. We prepared another box for the second cat. When she went into labor, she jumped into the first box. We put her in her own box. She went back. We put her back into the other box. She went back. Finally we just let her have her kittens in the already occupied box. We ended up with seven kittens with two mothers, and nobody knew or cared which belonged to who.