Hi folks
This one looks easy… X.org should just detect it, right?

dizzy-6:/home/leviathan # lspci | grep VGA
01:00.0 VGA compatible controller: NVIDIA Corporation GT200b [GeForce GTX 275] (rev a1)
02:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Caicos [Radeon HD 6450/7450/8450]

Well… It didn’t. It had to tinker a whole day until I got the right xorg.conf
I figured I share it with you:

dizzy-6:/home/leviathan # cat /etc/X11/xorg.conf
Section "Device"
        Identifier "radeon"
        Driver "radeon"
        BusID "PCI:2:0:0"
#       Option "ZaphodHeads" "HDMI-0"
        Screen 1 
EndSection

Section "Device"
        Identifier "nvidia-1"
        Driver "nouveau"
        BusID "PCI:1:0:0"
        Option "ZaphodHeads" "DVI-I-1"
        Screen 0
EndSection

Section "Device"
        Identifier "nvidia-2"
        Driver "nouveau"
        BusID "PCI:1:0:0"
        Option "ZaphodHeads" "DVI-I-2"
        Screen 0
EndSection

Section "Monitor"
        Identifier "DVI-I-1"
        Option  "RightOf" "DVI-I-2"
EndSection

Section "Monitor"
        Identifier "DVI-I-2"
EndSection

Section "Monitor"
        Identifier "HDMI-0"
EndSection

Section "Screen"
        Identifier "left_screen"
        Device "nvidia-1"
        Monitor "DVI-I-1"
        DefaultDepth 24 
#       SubSection "Display"
#               Virtual 1600 1200
#       EndSubSection
EndSection

Section "Screen"
        Identifier "middle_screen"
        Device "nvidia-2"
        Monitor "DVI-I-2"
        DefaultDepth 24
EndSection

Section "Screen"
        Identifier "hdmi_screen"
        Device "radeon"
        Monitor "HDMI-0"
        DefaultDepth 24
EndSection

Section "ServerLayout"
        Identifier "My Layout"
#       Option "Xinerama"
        Screen "middle_screen" 0 0
        Screen "left_screen" leftof "middle_screen"
        Screen "hdmi_screen" rightof "middle_screen"
EndSection