This should actually be rather easy with current nvidia drivers.
download the appropriate nvidia linux drivers for your card.
bring the system down to console only.
Log in as root & type:
sh <name of nvidia driver file>
Accept the license, say no to the ftp check, let the drivers compile a driver locally instead.
When it's finished, allow the installer to make the necessary changes to your xorg.conf file.
Once that's done, reboot, log in and run the nvidia configuration tool (under system tools) as root.
It should display both monitors, although one may be disabled.
apply the changes you want (resolution, xinerama) as required and click save.
The next time you restart X, it should be dual-head.
The one caveat I've found is that sometimes even if you run the nvidia config tool as root, because you're logged
in as a local user, it may not allow you to save the xorg file. In this case, just save it to your home directory, and as root, copy it into /etc/X11/ (after making a backup of the original xorg.conf)