I've done a little more experimenting:
Crossfire DISABLED - HW Decoding & Acceleration ENABLED - works just fine but it only used the 2ndary GPU when producing. The primary GPU had almost no load on it and the 2ndary GPU didn't show much of a load: 510MHz (of a 910MHz max) and generally around 10-20% load
Crossfire ENABLED - HW Acceleration DISABLED - HW Decoding ENABLED - system still blacks out and freezes
Crossfire ENABLED - HW Acceleration and Decoding DISABLED - seems to work just fine. Producing doesn't take that much longer either, perhaps 10-15% longer.