Re: Explain my prolem...
The system runs DirectX, the cards just talk to the software through DirectX, so it's not a different problem factor there.
More likely one of the issues is that you've either got outdated drivers, or the cards (the 7600's) weren't certified for HDCP, which they should be, so that's a little odd.
It sounds like the system isn't using your primary card for acceleration duties in the software. I have two 5870's in CrossFire and they play BluRays fine, but they're the only cards in my desktop system.