Right-click on the Catalyst icon in the notification area of the taskbar. Move to the first graphics card entry listed (mine has 'AMD Radeon HD 6900 series' listed 6 times, I guess one for each display output) , then AMD CrossfireX settings (this might show up under a different entry depending...
I confirmed it by using the setting in Catalyst that shows an icon in the top right when Crossfire is working. Exclusive full screen off - no icon, exclusive full screen on - crossfire icon (and a big jump in performance).
I've never come across an exclusive full screen setting in a game...
Crossfire, and the game in general, works without issues for me running 13.2 b7. I was having trouble getting crossfire working until I realised you have to keep the 'exclusive full screen' mode enabled.