A seção de comentários não permite grandes quantidades de texto. Então aqui está.
## This is the layout for the X server
Section "ServerLayout"
Identifier "Default Layout"
Screen 0 "screen1" 0 0
Option "Xinerama" "0"
EndSection
## This defines your graphics card, there should
## only be ONE such section for each card.
Section "Device"
Identifier "device1"
Driver "radeon"
BusID "PCI:1:0.0"
## Twinview lets you have one big desktop
## shared between the screens
Option "TwinView"
## This is where you need to findthe names of your two monitors
## and edit the line below accordingly. This is what I have
## on my system:
Option "metamodes" "DP-3: 1600x900, VGA-0: 1440x900, DP-2: 1920x1080, DP-1: 2560x1024"
Option "TwinViewOrientation" "RightOf"
Screen 1
EndSection
## I don't think these sections do anything unless you
## Identify the monitor somehow, they can probably be removed.
Section "Monitor"
Identifier "monitor1"
EndSection
Section "Monitor"
Identifier "monitor2"
EndSection
## This is where you define a single screen that stretches
## across both moitors.
Section "Screen"
Identifier "screen1"
Device "device1"
Monitor "monitor1"
SubSection "Display"
Depth 24
EndSubSection
EndSection