Nvidia keep their specifications private or under NDA, limiting capability.
You'll need to develop a working xorg.conf, the one you have working with Linux may be acceptable, as long as you point to the appropriate driver.
Your options with a GeForce 8xxx are to use either the
nv(4) driver or to use the
vesa(4) driver. Controlling individual screen outputs and resolution would be managed by
xrandr(1).
With the nv(4) driver, you have the "Dualhead" option for a larger side-by-side virtual desktop.