hmmm... i don't honestly know- however, i know many people buy two puppies at once- and i have never read that there are issues with human bonding. If it were me, I would do it when my dog was still a puppy because sometimes when dogs grow out of being a puppy- they have very little patience for puppy play and that could get in the way of their bonding. Also the younger your dog is, i feel, the less jealousy there will be from your dog or the need to protect "his" territory. But that's just my two cents
