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I used to have 2x 290x's when Catalyst was around and it did have the option to turn it on or off. I have a FuryX now but as far as I know Crossfire is set either globally or per game in Crimson Radeon Settings
Open Radeon Settings and go to Gaming tab, Under global settings you should be able to turn it off for all games or you can do it for each game separately in their profiles.
I believe that's how you do it though haven't had crossfire since Crimson has been out good luck hope this helps.