I'm getting an error when I startup GNOME when I switch my configuration to dual head display.

(EE) I810(0): You must have a MonitorLayout defined for use in a DualHead or Clone setup.
(EE) I810(1): Failed to setup second head due to primary head failure.
(EE) Screen(s) found, but none have a usable configuration.

Fatal server error:
no screens found

My configuration file looks like this. Any ideas would be most helpful!!!

# Xorg configuration created by system-config-display

Section "ServerLayout"
Identifier "Multihead layout"
Screen 0 "Screen0" LeftOf "Screen1"
Screen 1 "Screen1" 0 0
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
Option "Xinerama" "on"
Option "Clone" "off"
EndSection

Section "Files"

# RgbPath is the location of the RGB database. Note, this is the name of the
# file minus the extension (like ".txt" or ".db"). There is normally
# no need to change the default.
# Multiple FontPath entries are allowed (they are concatenated together)
# By default, Red Hat 6.0 and later now use a font server independent of
# the X server to render fonts.
RgbPath "/usr/X11R6/lib/X11/rgb"
FontPath "unix/:7100"
EndSection

Section "Module"
Load "dbe"
Load "extmod"
Load "fbdevhw"
Load "glx"
Load "record"
Load "freetype"
Load "type1"
Load "dri"
EndSection

Section "InputDevice"

# Specify which keyboard LEDs can be user-controlled (eg, with xset(1))
# Option "Xleds" "1 2 3"
# To disable the XKEYBOARD extension, uncomment XkbDisable.
# Option "XkbDisable"
# To customise the XKB settings to suit your keyboard, modify the
# lines below (which are the defaults). For example, for a non-U.S.
# keyboard, you will probably want to use:
# Option "XkbModel" "pc102"
# If you have a US Microsoft Natural keyboard, you can use:
# Option "XkbModel" "microsoft"
#
# Then to change the language, change the Layout setting.
# For example, a german layout can be obtained with:
# Option "XkbLayout" "de"
# or:
# Option "XkbLayout" "de"
# Option "XkbVariant" "nodeadkeys"
#
# If you'd like to switch the positions of your capslock and
# control keys, use:
# Option "XkbOptions" "ctrl:swapcaps"
# Or if you just want both to be control, use:
# Option "XkbOptions" "ctrl:nocaps"
#
Identifier "Keyboard0"
Driver "kbd"
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
EndSection

Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5"
Option "Emulate3Buttons" "yes"
EndSection

Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "LCD Panel 1024x768"
HorizSync 31.5 - 48.5
VertRefresh 40.0 - 70.0
Option "dpms"
EndSection

Section "Monitor"
Identifier "Monitor1"
VendorName "Monitor Vendor"
ModelName "LCD Panel 800x600"
HorizSync 31.5 - 37.9
VertRefresh 40.0 - 70.0
Option "dpms"
EndSection

Section "Device"
Identifier "Videocard0"
Driver "i810"
VendorName "Videocard vendor"
BoardName "Intel 855"
EndSection

Section "Device"
Identifier "Videocard1"
Driver "i810"
VendorName "Videocard Vendor"
BoardName "Intel 852"
BusID "PCI:0:2:0"
Screen 1
EndSection

Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 16
SubSection "Display"
Viewport 0 0
Depth 16
Modes "800x600" "640x480"
EndSubSection
EndSection

Section "Screen"
Identifier "Screen1"
Device "Videocard1"
Monitor "Monitor1"
DefaultDepth 16
SubSection "Display"
Viewport 0 0
Depth 16
Modes "800x600"
EndSubSection
EndSection

Section "DRI"
Group 0
Mode 0666
EndSection

Both the "Device" sections should be identical apart from identifier and the "Screen 0"/"Screen 1" lines
Try inserting the BusID and "Screen 0" into the first Device section.

Hope that helps