In Ubuntu Intrepid (with NVIDIA kernel module 177.xx) I ran a dual X-server setup (i.e. NOT a dual screen on the same server) so I could simultaneously use my Hauppauge tv-cards tv-out (with IVTV framebuffer) AND run a composited screen on my monitor. This worked perfectly.
Today, I upgraded to Ubuntu Jaunty, which by default uses kernel module 180.44 and found my screen is not composited anymore. From Xorg.0.log:
(EE) Failed to initialize GLX extension (Compatible NVIDIA X driver not found)
It has something to do with another X server running, because if I generate a single head xorg.conf using nvidia-xconfig, there are no problems.
Is the Nvidia driver now inherently incompatible with running two X servers, one of which not running on a Nvidia card?