>>15
On Windows, depending on your GPU drivers, you will be able to control each monitor as a separate Direct3D adapter. You can create a SwapChain for each monitor like that, and have multiple full screen windows.
With OpenGL on Windows and X11 alike (and probably OS X as well) you can do buffer sharing between contexts and either blit to monitors connected to different GPUs or create rendering contexts for all of them and render simultaneously.
At least in theory. None of these things actually work in real life, which is simply amazing considering that there are 3 huge hardware vendors all working on it; and they all suck monkey balls.