Once you have completed changing your XF86Config file, save it and your can now test your configuration :
root# > startx -- +xinerama |
Hopefully you can now enjoy X with multiple partners... er that is, Monitors.
The easiest way to have X start up with the Xinerama extentions is to add an option in the "ServerFlags section. as follows:
Section "ServerFlags" Option "Xinerama" "true" EndSection |
Now Xinerama should start every time you start X.