Wear her out= mate her too much. Which can can cause bald spots on their backs. When hormones kick in, she will begin to get a bare back. You need more than one hen, period. Raising them together has absolutley ha nothing to do with this. It is possible that they could of grew up to hate each other and never gotten along, but this case it was the opposite. Chickens have a pecking order. You don't really see this now, because you just have a pair. When you get more then will have a few scuffles to arrange a pecking order, it can't be avoided.