Please check "Chapter 14. Configuring GLX in Xinerama"
in the file "/usr/share/doc/NVIDIA_GLX-1.0/README.txt"
on how to setup a multi-monitor system with multiple GPUs.
There's also plenty of information on "Xinerama" in this
forum or on the web.
However, please note that Xinerama has a couple of limitations
which may or may not be a problem for you. For details on
this topic, please refer to