i have 2x hd 7750 in my pc, and i i'd lie to use them booth for gaming like under windows.
i have tried modding the atiogl.xml file in /etc/ati/, but adding games to it with the "<CrossFireCaps>0x00000100</CrossFireCaps>"
serious sam 3 just ended up with worse performance(and even to xserver crashes for tf2/css). i dont know who's job it is to make games work in crossfire,
but it would be cool if it would be taken into account on linux aswell.
the AMD driver team usually looks at enabling crossfire profile for games, and this has not been done yet for steam games.