You haven't named the profile you are using, nor the graphics driver. Both Crossfire and SLI tend to be quite problematic in some games, this one included, without a well-matched driver and profile.
Beyond that, the practicality of either Crossfire or SLI at low end levels is questionable, since by merely stepping up to a next level, spending less than the cost of the two lesser cards, you eliminate all such complications and get equal performance, if not better.