Not sure about how to disable Crossfire, but! I do remember I needed to disable ULPS (Ultra-Low Power State), what it basically does is disable cards that are not in use, so by disabling ULPS you make sure that all cards work at all times.

To disable ULPS:
1. Go to the registry ("regedit" in windows search - make sure you open it as admin).
2. Make sure "Computer" is highlighted and than go to Edit -> Find -> EnableULPS
3. Double click on EnableULPS on the right side and change its value to 0 (Hexadecimal base).

Restart PC and check to see that the registry was saved (again, make sure you access regedit as administrator).