Surprisingly, displays on the Mac Pro are only ever connected to one GPU. Apple expects you to use the second GPU for "offline rendering", which is essentially their own software version of SLI. It's not so useful for Resolume, as it would require passing textures to the second GPU, then rendering on the second GPU, then copying back to the main GPU for display.
http://devstreaming.apple.com/videos/ww ... l.pdf?dl=1
I've never heard of Resolume doing it's rendering on 2 cards in any machine. The 2nd card is just there to spit out what the 1st card renders. In most cases performance gets worse the second you use a single output on the 2nd card.
You always jump to point this out every chance you get, as if it's some sort of intrinsic flaw that Resolume users need to be made aware of. I'm curious to hear about alternatives to create an OpenGL context spanning all outputs, uploading texture data to multiple GPUs, utilising all GPUs for rendering, and all this without locking Resolume down to specific hardware and thereby going into a locked mediaserver paradigm.