AFAIK there's still an open question as to how the nMP routes the GPU's. Anandtech wasn't able to figure it out. Please correct me if anybody has done this, but I believe all we know is that there are two GPU's which multiplex to three TB2.0 busses, which are further multiplexed to six TB ports. I've done some checking. I downloaded the free GPU stress test too here. My nMP has three Cinema monitors ... Hex core D700 nMP Two TB which are on one bus An older 27" Cinema mDP on another bus A third bus which is dedicated to data (Pegasus2) I would hope that OS X would (somehow) dedicate one GPU to the older Cinema (center monitor), and the other GPU to the outer two monitor. However, running several instances of the GPU stress test seems to indicate otherwise. No matter on which monitor the windows are placed they perform equally FP wise (15-30 FPS on these tests). OK, so I launched the developer tool "Open GL Monitor" and looked at the AMDRadeonX4000GLDriver for FirePro D700 #1 and #2. The first sign is that the tool shows one of the GPU's as being "Offline". In Cocoa speak that means it's the "Offline Renderer", i.e. it's not routing to any display. Well, could mean something different, let's sample some data. There are many parameters to look at, I then checked video RAM used, Contexts and Surfaces. Uh-oh, these also showed that one of the cards was being heavily used, while the other was sitting idle (I can't tell which corresponds to which in the Renderer Info window). This seems to indicate that Apple simply plumbed one GPU to Thunderbolt to drive all your displays, and the other is always offline. The evidence is looking pretty clear. If they didn't then OS X is doing a poor job of allocating GPU's, simply taking the dumb approach of having one do all the heavy lifting while the other sits idle. What this means is, unless you use FCX extensively, one of your expensive GPU's is probably sitting idle most of the time. I'm disappointed, again Apple drops the ball. I'd like to know if the second GPU is plumbed to Thunderbolt at all? I'll do more tests later, Civ uses offline rendering too.