Re: Three monitors, NV420 quadro from dell with a 4 monitor dongle
Just to clarify the terminology, an "X server" is a single instance of the Xorg process. What you're describing sounds like two X screens on a single X server.
The feature you're looking for to combine multiple X screens into a single big X screen is called "Xinerama". You can enable it by running "nvidia-xconfig --xinerama". It has a few caveats, most notably that you lose the Composite extension, which means no desktop effects like wobbly windows or the desktop cube.