Eu finalmente consegui este trabalho, 2 Placas Gráficas (nVidia GTS 450) com ambas as telas em DVI.
Aqui está meu xorg.conf
:
Section "ServerLayout"
Identifier "Layout0"
Screen 0 "Screen0" 0 0
InputDevice "Keyboard0" "CoreKeyboard"
InputDevice "Mouse0" "CorePointer"
Option "Xinerama" "0"
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 "Acer S220HQL"
HorizSync 31.0 - 82.0
VertRefresh 56.0 - 76.0
Option "DPMS"
EndSection
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce 450 GTS"
BusID "PCI:1:0:0"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
Option "Stereo" "0"
Option "nvidiaXineramaInfoOrder" "CRT-0"
Option "metamodes" "GPU-56281125-01c7-ee85-b8b9-b3e5cf94b7c5.GPU-0.DVI-I-0: nvidia-auto-select +0+180, GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d.GPU-1.DVI-I-0: nvidia-auto-select +3600+0 {rotation=right}, GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d.GPU-1.DVI-I-3: nvidia-auto-select +1680+180"
Option "MultiGPU" "Off"
Option "SLI" "off"
Option "BaseMosaic" "on"
SubSection "Display"
Depth 24
EndSubSection
EndSection
Note que ele só faz referência a 1 GPU, por isso não precisa de outro X Screen.