Did you connect the crossfire bridges that came with the cards? Are both of your cards detected in Windows/Catalyst Control Center?
If you go into CCC and click on the "Performance" tab and then "AMD Overdrive" you should be able to select between both cards. (sorry for the big pic)
You can then click on "AMD Crossfirex configuration" and make sure that the box is checked that says "Enable Crossfirex".
If all of that is okay, then do you have the latest CAP installed? (Catalyst Application Profiles)
http://sites.amd.com/us/game/downloads/Pages/crossfirex-app-profiles.aspx
Those are the CFX profiles needed for all of the games and the CAP is updated regularly as new games are released.
edit: took screenshots from my setup and added them in.
starcraft: "I and every PS3 fanboy alive are waiting for Versus more than FFXIII.
Me since the games were revealed, the fanboys since E3." 
Skeeuk: "playstation 3 is the ultimate in gaming acceleration"
