It's an old game I know, but I'm currently playing The Witcher 2 Enhanced Edition (Steam). With Crossfire enabled on certain enemies the target selector and health bars dissapear making some fights near impossible. The only way to avoid this I've found is:
1) Disable Crossfire (drops fps to 40 in 2560x1440)
2) Enable Ubersampling (Also has a massive performance impact)
40fps from a single card is playable but with 2 the game is much smoother. Google suggests this was fixed ages ago but was re-introduced with a later driver.
GFX Card: XFX Radeon 7970 Black Edition (x2)
Driver : Catalyst 13.2 Beta 7 (fresh install - switched from GTX 570)
CPU / Mem : 16Gb, i7 2600k 3.4Ghz
OS: Win8 Pro 64bit
Mainboard: Gigabyte Z68XP-UD3P
Any help would be much appreciated!