How to fix my turtle?

So I have two turtles and one of them is a jerk and is always picking on the other one but one time the other one finally bit back. She bit him on the top of his neck and after that she kept biting there so now theres a big cut across his neck. We seperated the two so he could heal but he climbed out one time and messed it up. It's been about 2 months and it hasnt made any healing progress.... What do I do?