At this time, I'm just able to run a dual desktop with this kind of xorg.conf file .If you want use it don't forget to change screen resolution for the external display.

I'm looking for a sample "xorg.conf" able to toogle between two screens : I can't switch only on the external display. Please, send me an e-mail if you know the solution...

Thanks to zzlead a solution there : http://zzlead.blog.com/1650346/
(in french, maybe one day the blog will be in english ;) ), but for me impossible to add nvidia-settings package with nvidia-glx package...