Comment 3 for bug 308006

I am using NVIDIA. The problem is pretty much exactly as you describe. The resolutions available are only those available to xrandr.

I am using TwinView which I am aware does not use Xorg's Xinerama infrastructure for multiple screens. It does however allow the Xinerama X extension to be used to allow the window manager and other applications to be aware of the existence of the two screens that divide the common framebuffer. While it has been some time since I wrote or dealt with code at that level, if I recall correctly it should be possible for the low level code than handles setting screen resolutions to use Xinerama alongside GLX to discover the number and positions of the monitors.