I'be got two Nvidia cards, but Xorg activates only one of them. The following config is based on the nvidia configurator output:

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

Section "Module"
    Load           "dbe"
    Load           "extmod"
    Load           "type1"
    Load           "freetype"
    Load           "glx"
EndSection

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

Section "InputDevice"
    Identifier     "Keyboard0"
    Driver         "keyboard"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "HP LE2201w"
    HorizSync       24.0 - 83.0
    VertRefresh     50.0 - 76.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "Acer AL2017"
    HorizSync       30.0 - 82.0
    VertRefresh     56.0 - 76.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier  "Card0"
    Driver      "nvidia"
    VendorName  "nVidia Corporation"
    BoardName   "GeForce 6100 nForce 405"
    BusID       "PCI:0:13:0"
EndSection
Section "Device"
    Identifier  "Card1"
    Driver      "nvidia"
    VendorName  "nVidia Corporation"
    BoardName   "GeForce 8400 GS"
    BusID       "PCI:2:0:0"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "metamodes" "nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection
Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "TwinView" "0"
    Option         "metamodes" "nvidia-auto-select +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

What I see in the log file is:

(==) Log file: "/var/log/Xorg.0.log", Time: Fri Mar 19 11:08:08 2010
(==) Using config file: "/etc/X11/xorg.conf"
(==) ServerLayout "Layout0"
(**) |-->Screen "Screen0" (0)
(**) |   |-->Monitor "Monitor0"
(==) No device specified for screen "Screen0".
        Using the first device section listed.
(**) |   |-->Device "Card0"
(**) |-->Screen "Screen1" (1)
(**) |   |-->Monitor "Monitor1"
(==) No device specified for screen "Screen1".
        Using the first device section listed.
(**) |   |-->Device "Card0"
(**) |-->Input Device "Keyboard0"
(**) |-->Input Device "Mouse0"
(**) Option "Xinerama" "0"
(==) Automatically adding devices
(==) Automatically enabling devices

even though later on both cards are detected:

(--) PCI:*(0:0:13:0) 10de:03d1:1019:2601 nVidia Corporation C61 [GeForce 6100 nForce 405] rev 162, Mem @ 0xfb000000/16777216, 0xd0000000/268435456, 0xfc000000/16777216, BIOS @ 0x????????/131072
(--) PCI: (0:2:0:0) 10de:0422:0000:0000 nVidia Corporation G86 [GeForce 8400 GS] rev 161, Mem @ 0xf8000000/16777216, 0xe0000000/268435456, 0xf6000000/33554432, I/O @ 0x0000bc00/128, BIOS @ 0x????????/131072
[ --- some more logs --- ]
(II) Mar 19 11:08:10 NVIDIA(0): NVIDIA GPU GeForce 6100 nForce 405 (C61) at PCI:0:13:0
(II) Mar 19 11:08:10 NVIDIA(0):     (GPU-0)
[ --- some more logs --- ]
(II) Mar 19 11:08:12 NVIDIA(GPU-1): NVIDIA GPU GeForce 8400 GS (G86) at PCI:2:0:0 (GPU-1)

Unfortunately later on only one card is initialised and one screen is active. Xrandr shows only one screen too.

Any ideas on how to fix it?