I see this all the time with the litters we breed. Sometimes you would swear they are going to kill each other, but we have never had an injury. We occaisionally intervene when it looks like it's getting a little too rough, but my wife has also heard on a breeder's forum she's on that it's the puppies way of learning how hard to play without hurting the other. As johnch said, they are also working out who will be dominant.

I wouldn't worry too much about it. Just keep an eye on them as much as you can. It's a lot of work (but very rewarding) raising a litter.

Bruce