Bug#678011: gdm3: Squeeze->Wheezy regression. 2+ xorg devices Gnome3 launch fails

[Norbert Veber]

I recently ran into the same issue.  In my case I have a single nvidia
video card with two outputs connected to two monitors.  My prefered way
to set it up is so that they are two separate displays :0.0 and :0.1.
This worked great in debian 6 and gnome 2.x.  Each screen would get its
own gnome panels, and its own separate virtual desktops that could be
switched independantly.

In wheezy and gnome 3 this no longer works.  You are right in that its
not related to gdm, but rather the gnome shell.  You should see a
message like this in your ~/.xsession-errors:

gnome-session-is-accelerated: Zaphod mode not supported.

So basically you get the gnome-fallback session, and rather than
starting a gnome panel on each display as expected, it starts two of
everything on just one screen (as described in the original bug report).
It starts nothing on the second screen.

The workaround is to switch to xinerama/twinview mode instead which
extends a single display :0 to both monitors.  However it is a
regression since a configuration that used to work (and is by no means
exotic) is no longer supported.  Infact this used to be the only way to
work with multiple monitors until the advent of xinerama in XFree86 4.0.

Thanks,

Norbert