LinuxQuestions.org - GDM3 starts only on one X Screen of two

I have a multi-head environment:

Code:

Section "ServerLayout" 

    Identifier          "Default" 

    Screen              "Left" 0 0 

    Screen              "CentreAndRight" RightOf "Left" 

EndSection 







Section "Screen" 

    Identifier          "CentreAndRight" 

    Device              "hd6850" 

EndSection 



Section "Screen" 

    Identifier          "Left" 

    Device              "hd2000" 

EndSection 







Section "Device" 

    Identifier          "hd6850" 

    Driver              "radeon" 

    BusID              "PCI:1:0:0" 

EndSection 



Section "Device" 

    Identifier          "hd2000" 

    Driver              "intel" 

    BusID              "PCI:0:2:0" 

EndSection

As you can see I define two "Screen" sections since I have two video cards. The problem is that GDM3 starts GNOME session only on the first met Screen of the layout (I tried to swap them and the session was started on the other Screen).

Xorg.0.log says that there is no problem with any Screen. And indeed after I log in, the mouse pointer moves between one monitor with GNOME on it and two other black monitors. And for instance xrandr tool tells that there are indeed two screens and the monitors are identified correctly. So this makes me think that there's no problem with Xorg server but it is GDM3 which doesn't behave properly. Another strange thing is that if I want to get the value of DISPLAY variable on the Screen with GNOME I get ":0" instead of ":0.0". GDM logs also confirm that it is GDM who sets this value.

One more word about GDM2. Before upgrade to GNOME 3 I had no problem with GDM and could use all the three monitors simultaneously with the same xorg.conf So does GDM3 need to be configured differently?

BTW, I have default settings for GDM3.

I would be glad too to get a link to some really good and descriptive documents about GDM and X because the official ones seem to be very scanty and mostly low-level.