Dual head with xrandr 1.2 revisited

In this post, I discussed "clone" mode of dual head with xrandr 1.2. Now I got an extra monitor, and would like to use "xinerama" mode, wherein the built-in laptop LCD and the external monitor share a single virtual screen.

To set this up, I changed my /etc/X11/xorg.conf to add a monitor section for the external monitor:

Section "Device"   
  Identifier    "Intel 965GM"   
  Driver        "intel"   
  BusID         "PCI:0:2:0"   
  Screen        0   
  Option        "XAANoOffscreenPixmaps" "true"   
  Option        "DRI" "true"   
  Option        "monitor-LVDS" "Builtin"   
  Option        "monitor-VGA" "External" 
EndSection`