This is not a recommended practice and should be avoided. However, if you are in the situation of having two puppies from the same litter you need to approach this in a similar way to having a second dog but even more so. Each puppy needs individual socialisation. Puppies that are homed together need more socialisation with other dogs not less and they need to do this on their own. Each puppy should be walked on its own and given the opportunity to meet and socialise with other dogs as much as possible. You need to build a bond with each puppy that is greater than their bond with each other. In order to build a good relationship with your dogs, you need to make yourself more important than the other dog and this may mean interrupting or limiting the amount of play they have together. Solo is the key word. Train, play with and groom one puppy without the other being present in order to build an individual bond.