Is it normal for your puppies to bite each other?

This is normal puppy behavior and also a way of establishing dominance in their small pack. In order to avoid one of them becoming overly shy, it is a good idea that you begin taking them together for short daily walks on a leash to release some of that energy and also learn manners. Littermates will play-fight a lot and it can sound like they're going to kill each other, but blood is not drawn and no skin is broken. If it goes to actual injury (skin is penetrated) then it has gone too far. To break up a puppy fight you can probably scruff them (hold each pup by the shoulder skin firmly on the ground) and speak sternly. Think of how mother dog would have held a misbehaving pup with her teeth and a paw while she growled (stern warning).

Make sure to have separate training sessions with each pup as they grow up. A puppy class with off-lead, supervised socialization is a good idea so they learn from others what is not allowed.