There is very bad AMD Crossfire performance and poor utilization of the cards in the games current state. By default, the game does not work.
However, there is a way to force it to use Crossfire if you force it to use the Arma 2 profile, this does work and GPU usage does show that both cards are running, however performance is still not where it should be. However it is good to know that you can force it to work. Perhaps DayZ could get a properly configured profile by approaching AMD?