I recently tried to install Ubuntu on a desktop machine which was equipped with a dual screen setup. Having two screens and a ATI catalyst radeon X1950 PRO graphical video card. When installing Ubuntu from scratch it gave me initially the same screen on both screens and I wanted to extend the desktop so you could have one big screen to work on instead of clone the screen on both monitors.
To get rid of this problem I started to explore the /etc/X11/xorg.conf file stating how X will work with your monitor(s) and input devices. I have been reading some forums and guides where people stated how they achieved to get things working. I finally started with a promising howto guide and found myself in the position after a while where I could only have a screen on one single monitor. After some restore work and fixing some stuff I broke during this approach I tried it in a different way.
ATI is providing a proprietary drivers and a management tool. Even do I normally tend to the free and opensource drivers provided by the community I decided against it this time. Free as in freedom also means that you have the freedom not to choose the freedom op opensource, in this case I picked to give up some of this freedom. After installing the drivers and the management tool all was setup in a couple of minutes. So when you try to setup a dual screen in Ubuntu and you have a ATI card, the fastest way to do things is to use the ATI software.