Ric Moore composed on 2015-11-25 19:31 (UTC-0500):
as the only other way to
achieve what you want is xrandr. I tried to use/configure it once and
got a headache for my trouble.
If Mauro wants to try xrandr as a workaround until the root problem can be
found, and isn't familiar with xrandr methodology,
http://fm.no-ip.com/Share/setup may prove instructive. Here it lives and
works as /etc/X11/xinit/xinitrc.d/setup in openSUSE and Fedora. In
Debian/Mint/*buntu I have it also, but to make it work requires symlinking it
into /etc/X11/Xsession.d/, where I call it 95 setup on e.g. Wheezy.
What it contains is every xrandr setup I've ever needed that worked as
expected, all commented out except for whichever one line is needed. 4
displays at once I've yet to try, as all my Intel motherboards as yet have
provided no support for simultaneous use of PEG and IGP connections, and I've
never had inducement to try 2 PCIe cards at once or more than 2 displays at once.