I have 2 crossfired HD 7970's on Catalyst 13.9, and sometimes crossfire worked and sometimes it didn't.
When I first got AS I ticked the box in CCC to enable crossfire in games which do not have a CF profile. Now I force a profile via CCC specifically for this game. I force the profile 'AFR Friendly'
However crossfire was still tempremental, and using MSI Afterburner to monitor the cards, I could clearly see that GPU2 was often not used at all. It was like the system was running in 2D desktop mode ...... and that was precisely what was happening.
There was no difference whether I had the game set for full-screen or not. So I tried the keyboard shortcuts commonly used to get most games into fullscreen mode - LAlt-Enter, and that was it. The moment I did that the 2nd GPU kicked in.
So on my system (Windows 8.0 x64), it would appear that when I click the the DRIVE button and the track loads ups you are sitting in the pits, the new screen has actually lost focus. As it has lost focus, the system thinks I am not in fullscreen mode, and cuts the no.2 gpu ready for desktop use.
Every time I load up a track, I click the Assetto Corsa screen, and hit LAlt-Enter to enable crossfire
If I don't click the screen first, LAtl-Enter will produce an error sound, proving that Assetto hasn't got focus yet
Below is my current forced CF profile