To mirror monitors on an Asus Eee with Backtrack 4 Final:

1. Boot Backtrack

2. Enter this command
      startx

3. Plug in the external monitor

4. Open a Terminal window and enter this command:
      dpkg-reconfigure -phigh xserver-xorg

5. Log off

6. Enter this command
     X -configure

7. Enter this command
     startx

I got this from here:
http://www.thinkwiki.org/wiki/Xorg_RandR_1.2