Well, what have you tried so far? I have 2 boys and they've never been mean to each other with the exception of every now and then they might say something nasty to each other but that's very rare. They've always gotten along really well, it's kinda scary!!
Since I don't know what you've tried, and what hasn't worked, my suggestion would be that every time they hit, put them in opposite corners until they calm down. Then start taking stuff away. Television, toys, privileges, whatever is most important to them, and don't give it back until they can show they can be nice to each other. It may get to the point where they're sitting in their rooms without one toy to play with and no place to go and noone to play with, but if that's what it takes to get them to stop being so mean to each other then so be it!!
at 4:43 PM on Jun. 18, 2008