Hi and welcome to YT
I don't breed (I don't have the time, the knowledge, or the inclination!) but I have read many, many times in threads that breeding a very tiny yorkie (or other small dog for that matter) is dangerous and can result in expensive c-sections, if not the death of your dog. Maybe it would be best, for the health of your dogs to them neutered and enjoy them as wonderful pets. I wish u all the best with your two little ones and would love to see some pics!
xxx xxx