This is a tough one! I still have a bit of fighting between my girls, as well but it has become more and rarer. Do you feel like they are vying for your attention? It's possible that they may be jealous of your time spent with the other.
There are some very good articles written about stopping cats from urinating on the carpet. The first step after trying to find the reason, is to thoroughly clean the area using something like vinegar that will take out the scent. It is possible that she is using the same spot because she is attracted by the scent.