To disable Crossfire right-click your desktop and select "Catalyst Control Center". On the upper-left click "Graphics", then "CrossfireX". In the main area of the window, uncheck the "Enable Crossfire" option.
I believe, though I'm no expert, that if you have the same model GPU and same amount of memory, it should work. like a 7750 can be crossfired with a 7770 simply because the GPU model is the same, but the 7770 is Overclocked.