Я сделал вот так:
xrandr -s 1280x1024 (тут ставишь свое разрешение) будет показывать на обоих мониторах одно и то же.
xrandr -s 3200x1080 (тоже меняешь на свои цифры) влючается расширенный рабочий стол.
Xorg.conf
# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings: version 270.29 (buildd@samarium) Fri Feb 25 17:48:58 UTC 2011
#Section "Extensions"
# Option "AllowGLXWithComposite" "True"
# Option "Composite" "Enable"
#EndSection
Section "ServerLayout"
# Removed Option "Xinerama" "1"
Identifier "Default Layout"
Screen 0 "Screen0" 0 0
InputDevice "Keyboard0" "CoreKeyboard"
InputDevice "Mouse0" "CorePointer"
Option "Xinerama" "0"
EndSection
Section "Module"
Load "glx"
EndSection
Section "InputDevice"
# generated from default
Identifier "Keyboard0"
Driver "kbd"
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 "Monitor"
Identifier "Configured Monitor"
Option "DPMS" "false"
EndSection
Section "Monitor"
Identifier "Monitor0"
VendorName "Unknown"
ModelName "BenQ FP93GP"
HorizSync 31.0 - 83.0
VertRefresh 56.0 - 76.0
EndSection
Section "Monitor"
Identifier "Monitor1"
VendorName "Unknown"
ModelName "Philips FTV"
HorizSync 15.0 - 70.0
VertRefresh 48.0 - 62.0
EndSection
Section "Device"
Identifier "Configured Video Device"
Driver "nvidia"
Option "NoLogo" "True"
EndSection
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GTS 450"
Option "Coolbits" "1"
EndSection
Section "Device"
Identifier "Device1"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GTS 450"
BusID "PCI:1:0:0"
Screen 1
EndSection
Section "Screen"
Identifier "Default Screen"
Device "Configured Video Device"
Monitor "Configured Monitor"
DefaultDepth 24
EndSection
Section "Screen"
# Removed Option "TwinView" "0"
# Removed Option "metamodes" "DFP-0: nvidia-auto-select +0+0"
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
Option "TwinView" "1"
Option "TwinViewXineramaInfoOrder" "DFP-0"
Option "metamodes" "DFP-0: 1280x1024_60 +0+0, DFP-1: 1920x1080_60 +1280+0;DFP-0: 1280x1024_60 +0+0, DFP-1: 1280x1024 +0+0"
SubSection "Display"
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "Screen1"
Device "Device1"
Monitor "Monitor1"
DefaultDepth 24
Option "TwinView" "0"
Option "metamodes" "DFP-1: 1920x1080_60 +0+0"
SubSection "Display"
Depth 24
EndSubSection
EndSection
была изменена строчка
Option "metamodes" "DFP-0: 1280x1024_60 +0+0, DFP-1: 1920x1080_60 +1280+0
;DFP-0: 1280x1024_60 +0+0, DFP-1: 1280x1024 +0+0"
жирным выделено добавленное.
P.S. Видеокарта Nvidia, думаю для ATI не сильно будет отличаться.