The NVS420 integrates two GPUs behind a PCIe-Bridge and thus will
behave pretty much like two separate cards. Means: you can have
3D support up to the full resolution of 3840x2400 by enabling
Xinerama and there do exist a number of solutions to get the desired.
XineramaInfo. However, both, Xinerama and "Fakearama", come with
a number of problem. For details, please check the following threads:
My recommendation is to purchase a single GPU card with two outputs,
then purchasing two Matrox TripleHead2Go boxes which are capable
of splitting a 3840x1200 signal from a single output into 2x 1920x1200. So
with twinview, you can hook up to four displays onto a single GPU in a
3840x2400 (2x 2x 1920x1200) or a 7680x1200 (4x 1920x1200) single
framebuffer configuration. The desired XineramaInfo behaviour
can easily be archived with nvidias "XineramaInfoOverride" option.
For details, please check
BTW.: in your case, a pair of Matrox DualHead2Go boxes
(which are cheaper than the TH2Gs) should basically also
do the trick for you, but I don't know if the modelines from the
posts, linked above, will work for them.