LinuxQuestions.org - Two screens on separate graphic-cards. Possible on linux?

yeah... you are looking for Xinerama.... an example of it's use with xorg would be..
the contents of my xorg.conf file
Code:
# Xorg configuration created by system-config-display



Section "ServerLayout"

        Identifier    "Multihead layout"

        Screen      0  "Screen0" LeftOf "Screen1"

        Screen      1  "Screen1" 0 0

        InputDevice    "Mouse0" "CorePointer"

        InputDevice    "Keyboard0" "CoreKeyboard"

        Option            "Xinerama" "on"

        Option            "Clone" "off"

EndSection



Section "Files"

# RgbPath is the location of the RGB database.  Note, this is the name of the 

# file minus the extension (like ".txt" or ".db").  There is normally

# no need to change the default.



# Multiple FontPath entries are allowed (they are concatenated together)

# By default, Red Hat 6.0 and later now use a font server independent of

# the X server to render fonts.



        RgbPath      "/usr/X11R6/lib/X11/rgb"

        FontPath    "unix/:7100"

EndSection



Section "Module"

        Load  "dbe"

        Load  "extmod"

        Load  "fbdevhw"

        Load  "glx"

        Load  "record"

        Load  "freetype"

        Load  "type1"

        Load  "dri"

EndSection



Section "InputDevice"

# Specify which keyboard LEDs can be user-controlled (eg, with xset(1))

#        Option        "Xleds"                "1 2 3"



# To disable the XKEYBOARD extension, uncomment XkbDisable.

#        Option        "XkbDisable"



# To customise the XKB settings to suit your keyboard, modify the

# lines below (which are the defaults).  For example, for a non-U.S.

# keyboard, you will probably want to use:

#        Option        "XkbModel"        "pc102"

# If you have a US Microsoft Natural keyboard, you can use:

#        Option        "XkbModel"        "microsoft"

#

# Then to change the language, change the Layout setting.

# For example, a german layout can be obtained with:

#        Option        "XkbLayout"        "de"

# or:

#        Option        "XkbLayout"        "de"

#        Option        "XkbVariant"        "nodeadkeys"

#

# If you'd like to switch the positions of your capslock and

# control keys, use:

#        Option        "XkbOptions"        "ctrl:swapcaps"

# Or if you just want both to be control, use:

#        Option        "XkbOptions"        "ctrl:nocaps"

#

        Identifier  "Keyboard0"

        Driver      "kbd"

        Option            "XkbModel" "pc105"

        Option            "XkbLayout" "us"

EndSection



Section "InputDevice"

        Identifier  "Mouse0"

        Driver      "mouse"

        Option            "Protocol" "IMPS/2"

        Option            "Device" "/dev/input/mice"

        Option            "ZAxisMapping" "4 5"

        Option            "Emulate3Buttons" "yes"

EndSection



Section "Monitor"

        Identifier  "Monitor0"

        VendorName  "Monitor Vendor"

        ModelName    "Dell 1726T-HS/D1025HT"

        DisplaySize  330        240

        HorizSync    31.0 - 85.0

        VertRefresh  50.0 - 100.0

        Option            "dpms"

EndSection



Section "Monitor"

        Identifier  "Monitor1"

        VendorName  "Monitor Vendor"

        ModelName    "Dell P991"

        HorizSync    30.0 - 107.0

        VertRefresh  48.0 - 120.0

        Option            "dpms"

EndSection



Section "Device"

        Identifier  "Videocard0"

        Driver      "ati"

        VendorName  "Videocard vendor"

        BoardName  "ATI Mach64"

EndSection



Section "Device"

        Identifier  "Videocard1"

        Driver      "mga"

        VendorName  "Videocard Vendor"

        BoardName  "Matrox Millennium"

        BusID      "PCI:0:11:0"

EndSection



Section "Screen"

        Identifier "Screen0"

        Device    "Videocard0"

        Monitor    "Monitor0"

        DefaultDepth    16

        SubSection "Display"

                Viewport  0 0

                Depth    16

                Modes    "1024x768" "800x600" "640x480"

        EndSubSection

EndSection



Section "Screen"

        Identifier "Screen1"

        Device    "Videocard1"

        Monitor    "Monitor1"

        DefaultDepth    16

        SubSection "Display"

                Viewport  0 0

                Depth    16

                Modes    "1024x768"

        EndSubSection

EndSection



Section "DRI"

        Group        0

        Mode        0666

EndSection
hope this helps