It has been so long since I upgraded my laptop to Ubuntu Edgy Eft and dual screen does not work properly anymore. I tried several techniques but all failed. Anyway, I have just found a new hope in Gentoo Howto. By some modification the given xorg.conf in the howto, the dual screen works again.

Below is my xorg.conf.

#
# xorg config
#


# configuration to use
Section "ServerFlags"
#    Option     "DefaultServerLayout" "Single Local Layout"
    Option     "DefaultServerLayout" "Double Separate Desktop 1024 Layout"
EndSection


# two screens with two desktop, external 1024x768
Section "ServerLayout"
    Identifier  "Double Separate Desktop 1024 Layout"
    Screen      0 "Dual Local Screen" Below "Dual External1024 Screen"
    Screen      1 "Dual External1024 Screen"
    InputDevice "Generic Keyboard"
    InputDevice "Configured Mouse"
    InputDevice "Synaptics Touchpad"
EndSection


# one screen one desktop on local lcd
Section "ServerLayout"
    Identifier  "Single Local Layout"
    Screen      "Single Local Screen"
    InputDevice "Generic Keyboard"
    InputDevice "Configured Mouse"
    InputDevice "Synaptics Touchpad"
EndSection


# font files
Section "Files"
    FontPath    "/usr/share/X11/fonts/misc"
    FontPath    "/usr/share/X11/fonts/cyrillic"
    FontPath    "/usr/share/X11/fonts/100dpi/:unscaled"
    FontPath    "/usr/share/X11/fonts/75dpi/:unscaled"
    FontPath    "/usr/share/X11/fonts/Type1"
    FontPath    "/usr/share/X11/fonts/100dpi"
    FontPath    "/usr/share/X11/fonts/75dpi"
    FontPath    "/usr/share/fonts/X11/misc"
    # path to defoma fonts
    FontPath    "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection


# test what this does
Section "Extensions"
    # required for shadows, transparent windows etc.
    Option "Composite" "true"
EndSection


# modules to use
Section "Module"
    Load    "bitmap"
    Load    "dbe"
    Load    "ddc"
    Load    "dri"
    Load    "evdev"
    Load    "extmod"
    Load    "freetype"
    Load    "glx"
    Load    "int10"
    Load    "record"
    Load    "type1"
    Load    "vbe"
EndSection


# keyboard
Section "InputDevice"
    Identifier  "Generic Keyboard"
    Driver      "keyboard"
    Option      "CoreKeyboard"
    Option  "XkbRules"  "xorg"
    Option  "XkbModel"  "pc104"
    Option  "XkbLayout" "us"

    # required for non-us keyboard layouts
    # Option        "XkbLayout" "de"
    # Option        "XkbOptions"    "nodeadkeys"
EndSection


Section "InputDevice"
    Identifier  "Configured Mouse"
    Driver      "mouse"
    Option      "CorePointer"
    Option      "Device"        "/dev/input/mice"
    Option      "Protocol"      "ExplorerPS/2"
    Option      "ZAxisMapping"      "4 5"
    Option      "Emulate3Buttons"   "true"
EndSection

Section "InputDevice"
    Identifier  "Synaptics Touchpad"
    Driver      "synaptics"
    Option      "SendCoreEvents"    "true"
    Option      "Device"        "/dev/psaux"
    Option      "Protocol"      "auto-dev"
    Option      "HorizScrollDelta"  "0"
EndSection


# laptop lcd device section
Section "Device"
    Identifier  "Single LocalLCD Device"
    Driver      "i810" # change to vesa if it doesn't work
    BusID       "PCI:0:2:0"
    Screen      0

        Option      "PageFlip"      "true"
        Option      "VBERestore"        "true"
        Option      "DRI"           "true"
        Option      "AGPMode"       "4"
        Option      "AGPFastWrite"      "true"
        Option      "EnablePageFlip"    "true"
        Option      "DisplayInfo"       "false"
#        Option      "VideoRam"      "131072"
        Option      "VideoRam"      "65536"

    # ddc might complain about a wrong refreshrate
    Option      "DDC"       "false"
EndSection


# laptop lcd device section
Section "Device"
    Identifier  "Dual LocalLCD Device"
    Driver      "i810" # change to vesa if it doesn't work
    BusID       "PCI:0:2:0"

    ##### test this one
    #Option     "AGPFastWrite" "true"

    Screen      0

    # options for the i810 module only!
    # external monitor and local flat panel
    Option      "MonitorLayout" "CRT,LFP"
    # ddc might complain about a wrong refreshrate
    Option      "DDC"       "false"

    # use the next two lines to enable a cloned display for the external monitor
    # this might be used instead of dual head / xinerama (e.g. for presentations / beamer)
#   Option "Clone" "On"
    # refreshrate for the external monitor
#   Option "CloneRefresh" "60"
EndSection


# external vga device as dual display section
Section "Device"
    Identifier  "Dual VGAOUT Device"
    Driver      "i810"
    BusID       "PCI:0:2:0"
    Screen      1
    Option      "MonitorLayout" "CRT,LFP"

    ##### test this one
    #Option     "AGPFastWrite" "true"

    Option      "DDC"       "false"
#   Option      "Rotate"    "CCW"
EndSection


# laptop lcd monitor section
Section "Monitor"
    Identifier  "Dell X1 LCD Monitor"
    VendorName  "Dell"
    ModelName   "X1"
    Option      "DPMS"
    DisplaySize 338 203

    Mode        "1280x768"      # vfreq 59.870Hz, hfreq 47.776kHz
        DotClock    79.500000
        HTimings    1280 1344 1472 1664
        VTimings    768 771 778 798
        Flags       "-HSync" "-VSync"
    EndMode
    # 1280x1024 @ 60.00 Hz (GTF) hsync: 63.60 kHz; pclk: 108.88 MHz
    #Modeline "1280x1024_60.00"  108.88  1280 1360 1496 1712  1024 1025 1028 1060 -HSync +Vsync
EndSection


# external generic vga monitor section
Section "Monitor"
    Identifier  "Generic VGA Monitor"
    HorizSync       27 - 96
    VertRefresh     50 - 160
    VertRefresh     60
    DisplaySize 347 260 
    Option "DPMS"
    Modeline "1024x768_60.00" 65 1024 1032 1176 1344  768 771 777 806 -hsync -vsync
EndSection


# laptop 1280x768 lcd screen section 
Section "Screen"
    Identifier  "Single Local Screen"
    Device      "Single LocalLCD Device"
    Monitor     "Dell X1 LCD Monitor"
    DefaultDepth    24
    SubSection "Display"
        # test this
        Viewport    0 0

        Depth       24
        Modes       "1280x768" "1024x768" "800x600"
    EndSubSection
EndSection


# laptop 1280x768 lcd screen section 
Section "Screen"
    Identifier  "Dual Local Screen"
    Device      "Dual LocalLCD Device"
    Monitor     "Dell X1 LCD Monitor"
    DefaultDepth    24
    SubSection "Display"
        Depth       24
        Modes       "1280x768"
    EndSubSection
EndSection


# external 1024x768 vga screen section
Section "Screen"
    Identifier  "Dual External1024 Screen"
    Device      "Dual VGAOUT Device"
    Monitor     "Generic VGA Monitor"
    DefaultDepth    24
    SubSection "Display"
        Depth       24
        Modes       "1024x768"
        Virtual     1024 768
    EndSubSection
EndSection


# dri options
Section "DRI"
    Mode    0666
EndSection

I am not sure what the trick is but it works!

Tags: ubuntu, xorg.conf, 915gm, i810, dual head