Don't pull all of the nursing doe's pups before introducing the new ones. If they are around the same size, split the litter in half and try to get each doe to accept half. If one (or both) does is nursing a substantially larger litter, the larger babies will squash the newborns. So, you'll need to pull all but 3-4 of the original babies and place them with the other mom. Then gradually place the new babies in with the mom (3-4 at a time) until she's accepted all of them. As you put the new ones in, take one of the larger ones out. The whole process should take 1-2 hours. Keep them warm while you are waiting to place them with the new mother.