Can Windows switch GPUs while 3rd-party 3D apps are running? I'd be surprised if that worked.
The old NeXT cubes could do some impressive multi-"GPU" stuff, like having a window spanning two monitors, where one monitor was run by a plain black-and-white graphics board, and the other was the truecolor board with 2D acceleration and video I/O. The color half of the window was full true color, while the black-and-white half looked like it was designed for black-and-white, not just dithered down.
So, if anybody pulls off 3D switching on-the-fly, I would expect it to be Apple.