This article is from the Your New Puppy FAQ, by Cindy Tittle Moore with numerous contributions by others.
Puppies should not be separated from their mother and littermates
before 8 weeks of age. Many recommend 10 weeks minimum. This is
related to physical considerations such as weaning and psychological
considerations such as the puppy's readiness to leave the litter.
Many breeders believe it is best to NOT have two puppies together.
They tend to bond to each other and not to you and that can cause
serious problems when it comes time to train them. Having two puppies
needing housetraining at the same time can make that process go on for
much longer. This implies that you would not introduce a second dog
before the other six months old and properly trained.
There are always exceptions, of course, and there are many happy dogs
dogs that were littermates or otherwise puppies together out there.
 
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