Set maximum screen size, Ubuntu 12.04

I just setup my work station at my new workplace and found out that my Ubuntu 12.04 with ATI graphic card cannot output dual screen (two 1600x900) with message indicate that it exceed maximum screen size (1600x1600). After doing some search I found a fix.

I have to configure file xorg.conf in ~/etc/X11/ folder by adding subsection "display" part is section "screen" as follow (character in green):

[~/etc/X11/xorg.conf] 

Section "Screen"

Identifier "Default Screen"
DefaultDepth 24
    SubSection "Display"
        Virtual 3600 3600
    EndSubSection
EndSection
Section "Module"
Load "glx"
EndSection

Above code will change maximum size to 3600x3600 larger than what I really want (but it's fine). Save and reboot the system and I can use dual-monitor as I want.