In the Ubuntu french documentation (http://doc.ubuntu-fr.org/multi-ecran), they describe the process,
I traduce it, but I can't test since I don't have multiple displays:
.install the AMD driver (I don't know why they put it there, you can probably do nothing if the drivers are already installed)
.configure the multiple screens and save the configuration into a file
.copy the content of the configuration file into /etc/X11/Xsession.d/45x11-xrandr
I hope it will help you.