Gfilui

I have 2 GPUs and three monitors, but Ubuntu's X server can only work with either one GPU or the other GPU,but not both. How can I tell X server to compose a single screen from both GPUs ? I tried all the configurations possible and ended up with a single screen where mouse is working, but when moving mouse to the third monitor, the cursor changes from pointer to an "X" ,it does not accept windows moving onto it and it uses a black background. I just need somehow to tell X server that third monitor is also part of the screen, how should I do it?

This is my X11 config file:

niko@desk:~$ cat /etc/X11/xorg.conf
# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings: version 361.42 (buildd@lgw01-18) Tue Apr 5 14:33:28 UTC 2016

Section "ServerLayout"
 Identifier "Layout0"
 Screen 0 "Screen0" 0 0
 Screen 1 "Screen1" RightOf "Screen0"
 InputDevice "Keyboard0" "CoreKeyboard"
 InputDevice "Mouse0" "CorePointer"
# Option "Xinerama" "1"
EndSection

Section "Files"
EndSection

Section "InputDevice"
 # generated from default
 Identifier "Mouse0"
 Driver "mouse"
 Option "Protocol" "auto"
 Option "Device" "/dev/psaux"
 Option "Emulate3Buttons" "no"
 Option "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
 # generated from default
 Identifier "Keyboard0"
 Driver "kbd"
EndSection

Section "Monitor"
 # HorizSync source: edid, VertRefresh source: edid
 Identifier "Monitor0"
 VendorName "Unknown"
 ModelName "ViewSonic VA703-3Series"
 HorizSync 30.0 - 82.0
 VertRefresh 50.0 - 85.0
 Option "DPMS"
EndSection

Section "Monitor"
 # HorizSync source: edid, VertRefresh source: edid
 Identifier "Monitor1"
 VendorName "Unknown"
 ModelName "ViewSonic VA703-3Series"
 HorizSync 30.0 - 82.0
 VertRefresh 50.0 - 85.0
 Option "DPMS"
EndSection

Section "Device"
 Identifier "Device0"
 Driver "nvidia"
 VendorName "NVIDIA Corporation"
 BoardName "GeForce 8400 GS"
 BusID "PCI:7:0:0"
EndSection

Section "Device"
 Identifier "Device1"
 Driver "nvidia"
 VendorName "NVIDIA Corporation"
 BoardName "GeForce 210"
 BusID "PCI:8:0:0"
EndSection

Section "Screen"
 Identifier "Screen0"
 Device "Device0"
 Monitor "Monitor0"
 DefaultDepth 24
 Option "Stereo" "0"
 Option "nvidiaXineramaInfoOrder" "DFP-0"
 Option "metamodes" "VGA-0: nvidia-auto-select +1600+0, DVI-I-1: nvidia-auto-select +0+0"
 Option "SLI" "Off"
 Option "MultiGPU" "Off"
 Option "BaseMosaic" "off"
 SubSection "Display"
 Depth 24
 EndSubSection
EndSection

Section "Screen"
 Identifier "Screen1"
 Device "Device1"
 Monitor "Monitor1"
 DefaultDepth 24
 Option "Stereo" "0"
 Option "nvidiaXineramaInfoOrder" "CRT-1"
 Option "metamodes" "nvidia-auto-select +0+0"
 Option "SLI" "Off"
 Option "MultiGPU" "Off"
 Option "BaseMosaic" "off"
 SubSection "Display"
 Depth 24
 EndSubSection
EndSection

I have enabled Xinerama , disabled Composition extension in the config, and not it works. the login screen shows on all three monitors, but now I have got another problem, the X session wont start. I can start X by hand and output xterm to it from the console , but thats it. I think I should find a window manager that is designed for Xinerama.

EDIT:
Xinerama is 100% compatible with X. After looking into the problem I found out that GNOME or Ubuntu window managers do not work with Xinerama because they use composite extension which is disabled. So I installed Xcfe and now I have awesome window manager and it is times faster than the Ubuntu's default.