This sounds like your cat might have a tick. Cats are known to get more ticks then dogs, even indoor ones. Cats will claw themselves raw to try and get rid of the pest, but this can help the tick by forming openings for the tick to burrow into. With a magnifying glass, closely inspect your WHOLE cat with a small toothed comb. If a tick isn't the problem and your cat is biting itself, you can buy a nontoxic but bitter solution at the pet store for this problem. Rub a little on the cats tail and he will no longer want to bite himself. This will also stop the other cat from biting just in case. I hope this helps!