I've had disappointing results with multiple GPUs on my fairly beefy 680i Q6600 system:
Adding 3rd video card = *worse* performance?

It sounds like others have had more success rendering DX across multiple cards and monitors, but you're definitely running in lightly tested waters with that kind of configuration. I doubt that nVidia or ATI spend a lot of time worrying about bugs affecting 5 DX windows spread across 2-3 GPUs and 3+ monitors.