I have a Mac Pro that had the 2600XT in it, but it sucked, so I got an NVIDIA GTX 650Ti Boost, which has no boot screen and only works in 10.8.4(3?)+. I have 2 monitors, a regular one and a terrible $2 one that I normally use for Mail and Messages on the side, maybe an Xcode window. Can I use the GTX for one screen and the ATI for the other without system problems? Could this cause heat problems? I have the GTX in slot 1 and the ATI in slot 3. I also can't find the answer to how it renders things. If I use something that uses the GPU, which one does it use, the one whose screen it is on? What if I drag it across the screens? I've already crashed my Mac running GPUTest then dragging the window to the second display, so I'm worried. The point of this is so that I have a boot screen and can run Snow Leopard just in case. I guess it should also relieve the GTX of some duty. ---------- No longer a relevant question because I didn't want the clunker ATI in there and took it out. It was making blowdryer sounds and getting hot as usual and was probably wasting energy as well. That thing always makes you feel like your Mac Pro is about to explode. I'm still curious as to which GPU it uses when you have two, if anyone feels like answering.