Originally Posted by relix
Or is "Xinerama" not the same as "The nVidia Xinerama extension"?
Xinerama does two things:
a) Inside the X server: combine several video cards into a single display. This is non-trivial and doesn't work well with compositing (yet?).
b) Inside the protocol: report the screen boundaries to clients (mostly to the window manager) so they can react accordingly, for example maximizing windows only over one screen.
The Xinerama extension from nVidia doesn't do a) since the whole framebuffer is on one video card already. It just does b), reporting the screen boundaries to clients.