Which two cards are these? (Or you could attach nvidia-bug-report.log so I wouldn't have to ask questions =))
From the README (http://us.download.nvidia.com/XFree8...pter-14.html):
Using identical GPUs is recommended. Some combinations of non-identical, but similar, GPUs are supported. If a GPU is incompatible with the rest of a Xinerama desktop then no OpenGL rendering will appear on the screens driven by that GPU. Rendering will still appear normally on screens connected to other supported GPUs. In this situation the X log file will include a message of the form:
(WW) NVIDIA(2): The GPU driving screen 2 is incompatible with the rest of
(WW) NVIDIA(2): the GPUs composing the desktop. OpenGL rendering will
(WW) NVIDIA(2): be disabled on screen 2.