
When I had two litters born 4 days apart I kept them strictly apart even dividing the puppy area outside in half, I didn't want puppy owners to have any doubts which puppy came from which litter.
My mentor advised me to do this.
Years later I had two litters at the same time again, by this time there was 3 1/2 weeks between, so by the time the youngest were playing outdoors (not in a puppy pen) there were only two of the older ones left, and there is no way you could get them mixed up, with the older being twice the size.