I recently got a Sapphire R9 280x Vapor-x to crossfire with my Gigabyte Radeon HD 7950. OK, it seems to work - on some games it seems to be more of an issue though. But I can just disable crossfire for those games.
Anyways, I would like to disable crossfire by default since it seems that in my browser the screen flashes black every now and then, and when streaming videos I often get a green flash.
So I disabled crossfire and chose an AMD crossfire mode in Radeon Settings (Crimson Edition 15.12) for Black Ops 3. Now, BO3 should detect a multi-gpu environment and disable filmic antialiasing methods, but it doesn't - not in any of the profiles. Then only when I enable crossfire "globally" and launch the game does it actually detect and disable the problematic (for multi-gpu, ie) antialiasing methods. So this tells me that I cannot disable crossfire globally and have it activate on game launch. So I must either endure the black and green flashes or manually switch crossfire on and off?