I doubt they are truly "fighting." Puppies can get pretty rowdy and make some pretty crazy noises. There's gonna be yelping occasinally, especially if they are that young, and it's not necessarily a bad thing. They're teaching each other what is right and wrong & what's acceptable. You will know if they are really truly fighting. If you feel they're getting too rough, just separate them for a bit and let them cool down. You can see some pups in action here: