I'm using dual displays on my laptop Dell Latitude D600, with an
external display: Dell 1707FP
I had had to edit the xorg.conf file to get it working.
Here is my file: Cheers, Rene Pit
# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool,
using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf(5) manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades
*only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically
updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

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

Section "Module"
Load "i2c"
Load "bitmap"
Load "ddc"
Load "dri"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "vbe"
EndSection

Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "CoreKeyboard"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "gb"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/input/mice"
Option "Protocol" "ImPS/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

Section "InputDevice"
Driver "wacom"
Identifier "stylus"
Option "Device" "/dev/input/wacom"
Option "Type" "stylus"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "InputDevice"
Driver "wacom"
Identifier "eraser"
Option "Device" "/dev/input/wacom"
Option "Type" "eraser"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "InputDevice"
Driver "wacom"
Identifier "cursor"
Option "Device" "/dev/input/wacom"
Option "Type" "cursor"
Option "ForceDevice" "ISDV4" # Tablet PC ONLY
EndSection

Section "Device"
Identifier "ATI Technologies Inc Radeon R250 [Mobility
FireGL 9000]
"
Driver "ati"
BusID "PCI:1:0:0"
Screen 0
Option "DRI" "true"
Option "ColorTilling" "on"
Option "EnablePageFlip" "true"
Option "AccelMethode" "EXA"
Option "XAANoOffscreenPixmaps" "true"
Option "RenderAccel" "true"
Option "AGPMode" "8"
Option "AGPFastWrite" "on"
EndSection

Section "Device"
Identifier "ATI Technologies Inc Radeon R250 [Mobility
FireGL 9000]
2"
Driver "ati"
BusID "PCI:1:0:0"
Screen 1
EndSection

Section "Monitor"
Identifier "Laptop Monitor"
Option "DPMS"
EndSection

Section "Monitor"
Identifier "Dell Monitor"
Vendorname "Dell"
ModelName "1707FP Flat Panel Monitor"
HorizSync 30-81
VertRefresh 56-76
Option "DPMS"
EndSection

Section "Screen"
Identifier "Laptop Screen"
Device "ATI Technologies Inc Radeon R250 [Mobility
FireGL 9000]
"
Monitor "Laptop Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 4
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 8
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 15
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 16
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 24
Modes "1024x768"
EndSubSection
EndSection

Section "Screen"
Identifier "Dell Screen"
Device "ATI Technologies Inc Radeon R250 [Mobility
FireGL 9000]
2"
Monitor "Dell Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024"
EndSubSection
EndSection

Section "ServerFlags"
Option "Xinerama" "ON"
EndSection

Section "ServerLayout"
Identifier "Dualhead Layout"
Screen 0 "Laptop Screen" 0 0
Screen 1 "Dell Screen" RightOf "Laptop Screen"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
InputDevice "stylus" "SendCoreEvents"
InputDevice "cursor" "SendCoreEvents"
InputDevice "eraser" "SendCoreEvents"
InputDevice "Synaptics Touchpad"
EndSection

Section "DRI"
Mode 0666
EndSection

Section "Extensions"
Option "Composite" "false"
EndSection

You probably have to modify the file to your specific needs, but I
hope it gives you an idea!!!