GPUs that support TwinView (Chapter 11, Configuring Multiple Display Devices on One X Screen) can also be configured to treat each connected display device as a separate X screen.

While there are several disadvantages to this approach as compared to TwinView (e.g.: windows cannot be dragged between X screens, hardware accelerated OpenGL cannot span the two X screens), it does offer one advantage over TwinView: If each display device is a separate X screen, then properties that may vary between X screens may vary between displays (e.g.: depth, root window size, etc).

To configure two separate X screens to share one graphics card, here is what you will need to do:

First, create two separate Device sections, each listing the BusID of the graphics card to be shared and listing the driver as "nvidia", and assign each a separate screen:

    Section "Device"
        Identifier  "nvidia0"
        Driver      "nvidia"
        # Edit the BusID with the location of your graphics card
        BusID       "PCI:2:0:0"
        Screen      0
    EndSection

    Section "Device"
        Identifier  "nvidia1"
        Driver      "nvidia"
        # Edit the BusID with the location of your graphics card
        BusId       "PCI:2:0:0"
        Screen      1
    EndSection

Then, create two Screen sections, each using one of the Device sections:

    Section "Screen"
        Identifier  "Screen0"
        Device      "nvidia0"
        Monitor     "Monitor0"
        DefaultDepth 24
        Subsection "Display"
            Depth       24
            Modes       "1600x1200" "1024x768" "800x600" "640x480" 
        EndSubsection
    EndSection

    Section "Screen"
        Identifier  "Screen1"
        Device      "nvidia1"
        Monitor     "Monitor1"
        DefaultDepth 24
        Subsection "Display"
            Depth       24
            Modes       "1600x1200" "1024x768" "800x600" "640x480" 
        EndSubsection
    EndSection

(Note: You'll also need to create a second Monitor section) Finally, update the ServerLayout section to use and position both Screen sections:

    Section "ServerLayout"
        ...
        Screen         0 "Screen0" 
        Screen         1 "Screen1" leftOf "Screen0"
        ...
    EndSection

For further details, refer to the XF86Config(5x) or xorg.conf(5x) man pages.