how to configure xorg.conf for extended desktop?

I would like to setup the dual monitor settings where I can use two monitors as a single Desktop with single panel, single launcher and when some programs are maximized then it should span both the monitors (i.e fullscreen over both the monitors). I have ATI Raedon graphics cards and after searching it in the internet I found that by configuring the xorg.conf file I can setup the settings that I want. I have tried several ways to do it i.e using gksu amdcccle And setting MultiDisplay settings, but my requirement has not yet been met. ***NOTE:*** I have done this using Nvidia graphics but its seem that it doesn't work with ATI graphics card. After searching for a while I found the link which gave me hope that it can be done in ATI as well, According to this [link](http://phoronix.com/forums/showthread.php?18553-Ubuntu-9-04-amp-Catalyst-9-7-%28fglrx%29) . xrandr must be disabled and AMD catalyst center will give the extra option of Big Desktop, to set it I used sudo aticonfig --set-pcs-str="DDX,EnableRandR12,FALSE" Then I restart X as per that [link](http://phoronix.com/forums/showthread.php?18553-Ubuntu-9-04-amp-Catalyst-9-7-%28fglrx%29) . But this command gives this output to the stdout: > SetPcsStr is exclusive, other options will not be used. And I didn't find any extra configuration in my xorg.conf file. And I didn't even get the Big Desktop option in the AMD Catalyst center's Display manager. There is still three options i.e: Single Desktop Cloned Display Multiple Display