dual monitors

You don't mention which graphics card you have.

If it's nvidia

http://www.ubuntugeek.com/dual-monitors-with-nvidia.html

It's important to know the graphics card because the approach actually differs.

If you have a graphics card which supports xrandr 1.2, you can use the set resolution utility under System -> Preferences. This is definitly the easyest and most user friendly way to fix this. Unfortunatly, currently very few graphics card support this feature. I only know that the cards using the Intel driver. If the total resolution of the monitors exceeds 2048 in width or height, you will need to change some settings in xorg.conf to make it work correctly. Please check out http://www.thinkwiki.org/wiki/Xorg_RandR_1.2 for more information about this.

Nvidia currently requires that you use the proprietary driver and configure xorg - this is somewhat easier to do with the tool nvidia-settings. You should run nvidia-settings from a terminal by typing: sudo nvidia-settings

For ATI I will be of little-to-no help, since I don't have hands-on experience on setting it up. But you should be able to achieve this by setting up Xinerama. Googling for ubuntu xinerama points me in this direction: http://ubuntuforums.org/showthread.php?t=221174

I hope this helps.

Regards, Trygve