If when you try to set a dual monitor with xrandr obtain the message:
$ xrandr: screen cannot be larger than widthXheight (desired size mywidthXmyheight)
You can solve creating a new file in:
$ /usr/share/X11/xorg.conf.d/
e.g.:
$ sudo nano /usr/share/X11/xorg.conf.d/50-default-screen.conf
With the following content:
Section "Screen"
Identifier "Default Screen"
SubSection "Display"
Virtual 4096 4096
EndSubSection
EndSection
Then restarting the session!
Enjoy.
