I can certainly relate. They still fight but now it is just in "fun" to show off their teen age strength.
I read an article when they were little and the advice sometimes helped me. Can't remember the psychologist who wrote it but he had recently passed away and this was included in a story about him.
1. Don't take sides. Punish equally and refuse to listen to the reason/cause/start of the fight. ( I would walk over, turn off TV and send both to their rooms, for example)
2. Kids fight only when they have an audience. If they are picking on each other give yourself a timeout. Really, I noticed my boys would play all day together for hours in the basement with their action figures but when they came up around me, BAM.
3. Keep your calm. If you need to, go in the bathroom and take a bath, in your bedroom, just walk away. It is kind of a shock to them that you won't stay there and participate that they often forget to continue the argument.
This too will pass. Honest. Just try to keep it in perspective. They have to learn how to argue, win or lose, so they can handle conflict as adults. Don't you know people who get really weird when they are confronted by conflict? I do. I guess that is why families with lots of kids seem to be pretty well adjusted. If you look in nature, baby bears, tiger cubs, monkeys all tussle and wrestle.