If girl A is a more experienced rider, and girl A's horse is a more difficult to handle horse, while girl B is less likely to be capable of handling girl A's horse, since girl B can't even warm up her own horse....then it's perfectly fair. But that might not be the case, there are far too many variables here.
God, do you KNOW how many times I had to re-read that to make sure I had my A's and B's in the right places? *lol*
If Girl A and Girl B are comparable level riders, on comparable level horses, then each just ride their own horse, if it's an issue. Goodness me.