Configuring ATI CrossFireX™ for ATI Radeon™ X1300 Series and Later
Use the Configure page to enable and configure dual, tri, and quad-GPU ATI CrossFireX configurations. When ATI CrossFireX support is enabled, 3D games and applications automatically uses ATI CrossFireX technology if the driver determines it to be the optimal solution.
When ATI Catalyst™ A.I. is enabled, the optimal rendering mode available for the 3D games and applications is automatically selected. You can also adjust Super Anti-Aliasing for ATI CrossFireX using the 3D aspects in the Graphics Settings tree. When ATI Catalyst A.I. is disabled, 3D games and applications do not use ATI CrossFireX technology. If necessary, you can disable this feature to temporarily suppress ATI CrossFireX support for 3D games and applications.
ATI CrossFireX™ Rendering Modes
An ATI CrossFireX system has four possible display modes:
Alternate Frame Rendering
The first three are performance-oriented modes, while Super Anti-aliasing is a quality-oriented mode. Each mode uses a different method for dividing the workload required to render a 3D image across multiple GPUs. The ATI Catalyst™ display driver automatically selects the best of the three performance modes when a 3D application is started.
Tip: You can alternatively choose to improve image quality by selecting one of the Super Anti-aliasing modes (levels above 6X) in ATI Catalyst™ Control Center > Advanced > Graphics Settings > 3D > Anti-Aliasing.
I don't have crossfire but it looks like you simply enable it and either let Catalyst AI handle the mode it uses or you can manually set it to one if AI is disabled.