The only way we have found that works is biting back. Once they are bitten back it should work. I know its mean n not one people want to try but its affective. All kids that have bit n been bitten back have stopped.
I'd give them a dab of hot sauce when they do. Not enough to fully burn their mouths but just enough to get the point across. When I was younger an aunt of mine washed both my cousin's mouth and mine with soap on a toothbrush when we bit each other. Didnt do it again.
Try a little ceyenne pepper or a touch of dish soap on their lips and/or tongue. Biting is a very bad habit and leads to other forms of anger. Next thing they will be hitting each other over the head with something and someone could get hurt. You might also check with the doctor to see if he recommends something that you can use that is safe. Good Luck