according to this article: http://www.2030.tk/wiki/Dual_monitor_on_Ubuntu_7.10

run xrandr -q

to get info about the screens connected. If you have two screens at 1280×1024, you’ll want a virtual resolution of 2560×1024 ..

set the Virtual in Xorg.conf. I inserted the Virtual line into mine here:

Section “Screen”
Identifier    ”Default Screen”
Device        ”Configured Video Device”
Monitor        ”Configured Monitor”
Defaultdepth    24
SubSection “Display”
Depth    24
Virtual    2560    1024
Modes        ”640×480@60″
EndSubSection
EndSection

run

xrandr --output DVI-0 --auto --left-of VGA-0

Where DVI-0 and VGA-0 are the identifies for your screens as noted by xrandr -q

One option for having dual head mode at start up is to write the above command into a shell script and have it run by the session manager in gnome/kde at startup.

My xorg.conf file:

xorg.conf