I had a similar issues, so much so I decided to rebuild the PC.
Now my Crossfire 6870's are on a brand new build, beta 13.2 7 For whatever odd reason if I start the game from a PC cold boot with crossfire enabled the screen just goes blank(have to power off). However if I turn Crossfire off after power off/restart, start the game it runs fine. If I quit the game turn Crossfire back on and start the game it runs fine. Since I had similar issue with this before the rebuild i suspect driver/and or application issues. No other games I've tried have issues on the cold boot.
When it does run its been doing better. The frame rates seem better crossfired but since I am only running it on the high settings with Tress on at 1080P. the in game benchmark was minimun 32 Max 62 Average 52