H.A.
Both my parents remarried and so I grew up with 2 "real" (blood) sisters, as well as 2 step-sisters, and 3 step-brothers (busy houses!) There were times we HATED each other. It had nothing to do with the fact that we were step siblings, but that we were just siblings. Especially us girls! We could be vicious and mean. My parents would seperate us, make us go do our own thing and stay away from each other until we could get along.
I think the best thing to do would be to offer consequences for their disrepect toward each other. They don't have to be best friends, but they do have to be held accountable for their actions. Hold them to the "If you can't say anything nice, don't say anything at all" rule, and start taking away privliges when they fight. They are both old enough to walk away and turn the other cheek.