That's a known limitation: the very moment "true" Xinerama is
enabled (because of two video cards), then the "faked"
Xinerama extension of the nvidia driver is disabled.
There are a number of patches available where you can
set the XineramaInfo to arbitrary values, i.e. this one:
However, the problem is that this solution requires patching
and re-compiling parts of the X-server, which might not be an
easy task for most common users. (also, i'm not sure if
there any patches around for the recent Xorg releases).
In general, Xinerama has a number of limitations, i.e. it
is sort of incompatible with the composite and randr
extensions (required for Compiz). Apart from other
software bugs, two cards in one PC are always a source
of problems, i.e. PSU breakdown or overheating.
For this reason, I switched to another hardware solution
that allows to run six LCDs up to 5040x2100 (2x3x1680x1050)
or four LCDs at 3840x2400 (2x2x1920x1200) from a single
However, this variant isn't cheap ...