After playing around for a while, I've managed to get both cards run at 392 Mhashes/s.  It seems that I don't entirely understand the interaction between Trixx and Catalyst Control Center.  I thought Trixx would control both cards when the "Synchronize cards in Multi-GPU config" was chosen.  However, this seems not to be the case.  The second card is not paying attention to Trixx controls, only CCC controls; yet the first card pays attention to both - they have to be the same in both apps or the performance of the first card suffers.

Also, it seems that mhash rate takes a hit whenever the fan speed changes; having the default "custom" setting in trixx (C=%) resulted in worse performance than just keeping a steady clock speed.