I'm using KUbuntu 9, and two monitors: CRT+DVI. Nvidia's TwinView works fine, but enabling Xinerama extension disables compositing in KDE.
Is there a way to enable compositing (desktop effects) with Xinerama? Why does this happen?
Compositing requires certain graphical extensions (most commonly GLX) to be available on your graphics card and enabled in your X server. However, few drivers have simultaneous support for the technologies that drive Xinerama and GLX - enabling one tends to disable the other.
Recent (GeForce 7 or so and newer) Nvidia cards are capable of enabling such functionality, if the GPUs are similar enough. If not, the official Nvidia driver will only disable GLX extensions on the card that doesn't support Xinerama+GLX, rather than disabling it across all screens. AFAIK, the open-source nv driver doesn't support Xinerama+GLX at all (or if it does, it's subject to the same limitations as the official nvidia driver).
If there's absolutely no way for you to get the two working nicely on your hardware, you might want to take a look at the mesa library - it's a software 3D rendering tool that replaces hardware GLX and allows compositing to happen, albeit with a pretty big performance hit.
You usually do not enable the usual Xorg Xinerama extension when using TwinView, since the nVidia driver already provides a Xinerama extension compatible with TwinView.
From the driver's README: