Hi,
Well, I use an ATI card, so it is for the archive ... may be it is of any help

for the nvidia users?!
1) See my xorg.conf below, the only line I had to change here to use dual head

resolution, is in the screen section, I added:  "Virtual 3840 1200"
2) I added this line in my .xinitrc:

xrandr --fb 1920x1200 --output DVI-0 --left-of DVI-1 &
Now I have  multihead working great, no need to add more monitor, device and

screen section anymore etc .... isn't that easy? ;-)
My Xorg.conf:
Section "ServerLayout"

        Identifier     "X.org Configured"

        Screen      0  "Screen0" 0 0

        InputDevice    "Mouse0" "CorePointer"

        InputDevice    "Keyboard0" "CoreKeyboard"

EndSection
Section "Files"

        #RgbPath      "/usr/X11R6/share/X11/rgb"

        ModulePath   "/usr/X11R6/lib/modules"

        FontPath     "/usr/X11R6/lib/X11/fonts/misc/"

        FontPath     "/usr/X11R6/lib/X11/fonts/TTF/"

        FontPath     "/usr/X11R6/lib/X11/fonts/OTF"

        FontPath     "/usr/X11R6/lib/X11/fonts/Type1/"

        FontPath     "/usr/X11R6/lib/X11/fonts/100dpi/"

        FontPath     "/usr/X11R6/lib/X11/fonts/75dpi/"

EndSection
Section "Module"

        Load  "GLcore"

        Load  "dbe"

        Load  "extmod"

        Load  "glx"

        Load  "record"

        Load  "xtrap"

        Load  "dri"

        Load  "freetype"

        Load  "type1"

EndSection
Section "InputDevice"

        Identifier  "Keyboard0"

        Driver      "kbd"

EndSection
Section "InputDevice"

        Identifier  "Mouse0"

        Driver      "mouse"

        Option      "Protocol" "wsmouse"

        Option      "Device" "/dev/wsmouse"

        Option      "ZAxisMapping" "4 5 6 7"

EndSection
Section "Monitor"

        #DisplaySize      520   320     # mm

        Identifier   "Monitor0"

        VendorName   "BNQ"

        ModelName    "BenQ G2400W"

 ### Comment all HorizSync and VertRefresh values to use DDC:

        HorizSync    31.0 - 94.0

        VertRefresh  50.0 - 85.0

        Option      "DPMS"

EndSection
Section "Device"

        ### Available Driver options are:-

        ### Values: : integer, : float, : "True"/"False",

        ### : "String", : " Hz/kHz/MHz"

        ### [arg]: arg optional

        #Option     "NoAccel"                   # []

        #Option     "SWcursor"                  # []

        #Option     "Dac6Bit"                   # []

        #Option     "Dac8Bit"                   # []

        #Option     "BusType"                   # []

        #Option     "CPPIOMode"                 # []

        #Option     "CPusecTimeout"             # 

        #Option     "AGPMode"                   # 

        #Option     "AGPFastWrite"              # []

        #Option     "AGPSize"                   # 

        #Option     "GARTSize"                  # 

        #Option     "RingSize"                  # 

        #Option     "BufferSize"                # 

        #Option     "EnableDepthMoves"          # []

        #Option     "EnablePageFlip"            # []

        #Option     "NoBackBuffer"              # []

        #Option     "DMAForXv"                  # []

        #Option     "FBTexPercent"              # 

        #Option     "DepthBits"                 # 

        #Option     "PCIAPERSize"               # 

        #Option     "AccelDFS"                  # []

        #Option     "DDCMode"                   # []

        #Option     "IgnoreEDID"                # []

        #Option     "DisplayPriority"           # []

        #Option     "PanelSize"                 # []

        #Option     "ForceMinDotClock"          #

        #Option     "ColorTiling"               # []

        #Option     "VideoKey"                  # 

        #Option     "RageTheatreCrystal"        # 

        #Option     "RageTheatreTunerPort"      # 

        #Option     "RageTheatreCompositePort"  # 

        #Option     "RageTheatreSVideoPort"     # 

        #Option     "TunerType"                 # 

        #Option     "RageTheatreMicrocPath"     #

        #Option     "RageTheatreMicrocType"     #

        #Option     "ScalerWidth"               # 

        #Option     "RenderAccel"               # []

        #Option     "SubPixelOrder"             # []

        #Option     "ShowCache"                 # []

        #Option     "DynamicClocks"             # []

        #Option     "VGAAccess"                 # []

        #Option     "ReverseDDC"                # []

        #Option     "LVDSProbePLL"              # []

        #Option     "AccelMethod"               #

        #Option     "DRI"                       # []

        #Option     "ConnectorTable"            #

        #Option     "DefaultConnectorTable"     # []

        #Option     "DefaultTMDSPLL"            # []

        #Option     "TVDACLoadDetect"           # []

        #Option     "ForceTVOut"                # []

        #Option     "TVStandard"                #

        #Option     "IgnoreLidStatus"           # []

        #Option     "DefaultTVDACAdj"           # []

        #Option     "Int10"                     # []

        Identifier  "Card0"

        Driver      "radeon"

        VendorName  "ATI Technologies Inc"

        BoardName   "RV370 5B60 [Radeon X300 (PCIE)]"

        BusID       "PCI:1:0:0"

EndSection
Section "Screen"

        Identifier "Screen0"

        Device     "Card0"

        Monitor    "Monitor0"

        DefaultDepth 24

        SubSection "Display"

                Viewport   0 0

                Depth     1

        EndSubSection

        SubSection "Display"

                Viewport   0 0

                Depth     4

        EndSubSection

        SubSection "Display"

                Viewport   0 0

                Depth     8

        EndSubSection

        SubSection "Display"

                Viewport   0 0

                Depth     15

        EndSubSection

        SubSection "Display"

                Viewport   0 0

                Depth     16

        EndSubSection

        SubSection "Display"

                Viewport   0 0

                Depth     24

                Modes "1920x1200"

                Virtual 3840 1200

        EndSubSection

EndSection