RandR 1.2 currently only works on a single X screen (a real one, not a Xinerama screen). You can get exactly the same "fakerama" behavior with the nvidia driver today with TwinView. To span multiple GPUs, you still need the real Xinerama extension, which still disables RandR. See hw/xfree86/modes/xf86RandR12.c in the xserver tree:
xf86RandR12Init (ScreenPtr pScreen)
/* XXX disable RandR when using Xinerama */
(PanoramiX is the old historical name for Xinerama).
RandR 1.2 support in the nvidia driver is in the works, but don't expect it to magically make RandR + real Xinerama work.