I've been meaning to post this sicne so many are having that "flickering" problem With 2+ vid cards . I have foudn 2 workarounds. Neither are the bonehead idea to shut off one card.
1st play in window mode, make the window big enough and it is like fullscreen, but without the flickering. I personally play all my games in window so i didn't notice I had the same issue in fullscreen. Ati crossfire plays well in window mode and if you have crossfire you prolly have quad core or better which can handle the extra strain.
2nd option, and I didn't like this one. Disable BOTH Catalyst AI AND LCD overclock/boost. This, with v syn, will remedy the flickering, however. It will cause other, abnormalities which I'm not fond of.
Again these are work arounds till crytek fix's the issue. Crytek don't call me, call ati and get it done, I could do a fix in 20 min if I had your code and I'm a layman.