I have a Matrox G550 AGP and a PCI G450 ina Gigabyte board running Redhat 8,..I have both video cards installed and am having a little trouble getting both LCD's to sync and work when x windows is started. Screen 0 initializes but when Screen 1 initializes,..it has alot of horizontal lines distorting the picture. I realize that either card would probably do it by itself but I need to get it working with both cards in there. It looks as if the refresh rate or driver or something is making it flakey,..I wish I knew. I have been to numerous sites and it seems everyone has their little way of editing their XF86Config file. I wanted to post my config file in hopes that some genious out there could pinpoint my error and give a little help. I havent been using Redhat long,..only @ 2 months daily. Im sorry if I have violated any rules by posting this big of a message... =( It was kind of thrown together by using others peoples posts and recommendations,..but something is not right. Thanks for your time.

Section "ServerLayout"
Identifier "XFree86 Configured"
Screen 0 "Screen0" 0 0
Screen 1 "Screen1" RightOf "Screen0"
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
Option "Xinerama"
EndSection

Section "Files"
RgbPath "/usr/X11R6/lib/X11/rgb"
FontPath "unix/:7100"
# ModulePath "/usr/X11R6/lib/modules"
# FontPath "/usr/X11R6/lib/X11/fonts/misc/"
# FontPath "/usr/X11R6/lib/X11/fonts/Speedo/"
# FontPath "/usr/X11R6/lib/X11/fonts/Type1/"
# FontPath "/usr/X11R6/lib/X11/fonts/CID/"
# FontPath "/usr/X11R6/lib/X11/fonts/75dpi/"
# FontPath "/usr/X11R6/lib/X11/fonts/100dpi/"
EndSection

Section "Module"
Load "dbe"
Load "dri"
Load "extmod"
Load "glx"
Load "speedo"
Load "type1"
EndSection

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

Section "InputDevice"
Identifier "Mouse0"
# Modified by mouseconfig
Driver "mouse"
Option "Device" "/dev/mouse"
Option "Protocol" "PS/2"
Option "Emulate3Buttons" "yes"
Option "ZAxisMapping" "4 5"
EndSection

Section "Monitor"
#DisplaySize 320 240 # mm
Identifier "Monitor0"
HorizSync 31.0 - 82.0
VertRefresh 55.0 - 85.0
Option "dpms"
EndSection

Section "Monitor"
Identifier "Monitor1"
HorizSync 31.0 - 82.0
VertRefresh 55.0 - 85.0
Option "dpms"
EndSection

Section "Device"
### Available Driver options are:-
### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
### <string>: "String", <freq>: "<f> Hz/kHz/MHz"
### [arg]: arg optional
Option "SWcursor" "True" # [<bool>]
Option "HWcursor" "Off" # [<bool>]
#Option "PciRetry" # [<bool>]
#Option "SyncOnGreen" # [<bool>]
Option "NoAccel" "True" # [<bool>]
#Option "ShowCache" # [<bool>]
#Option "Overlay" # [<str>]
#Option "MGASDRAM" # [<bool>]
#Option "ShadowFB" # [<bool>]
#Option "UseFBDev" # [<bool>]
#Option "ColorKey" # <i>
#Option "SetMclk" # <freq>
#Option "OverclockMem" # [<bool>]
#Option "VideoKey" # <i>
#Option "Rotate" # [<str>]
#Option "TexturedVideo" # [<bool>]
#Option "Crtc2Half" # [<bool>]
#Option "Crtc2Ram" # <i>
Option "Int10" "True" # [<bool>]
#Option "AGPMode" # <i>
#Option "DigitalScreen" # [<bool>]
#Option "TV" # [<bool>]
#Option "TVStandard" # [<str>]
#Option "CableType" # [<str>]
#Option "NoHal" # [<bool>]
#Option "SwappedHead" # [<bool>]
#Option "DRI" # [<bool>]
Identifier "Card0"
Driver "mga"
VendorName "Matrox"
BoardName "MGA G550 AGP"
BusID "PCI:1:0:0"
EndSection

Section "Device"
### Available Driver options are:-
### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
### <string>: "String", <freq>: "<f> Hz/kHz/MHz"
### [arg]: arg optional
#Option "SWcursor" "True" # [<bool>]
Option "HWcursor" "Off" # [<bool>]
#Option "PciRetry" "True" # [<bool>]
#Option "SyncOnGreen" "True" # [<bool>]
Option "NoAccel" "True" # [<bool>]
#Option "ShowCache" # [<bool>]
#Option "Overlay" # [<str>]
#Option "MGASDRAM" "True" # [<bool>]
VideoRam 32768
#Option "ShadowFB" # [<bool>]
#Option "UseFBDev" # [<bool>]
#Option "ColorKey" # <i>
#Option "SetMclk" # <freq>
#Option "OverclockMem" # [<bool>]
#Option "VideoKey" # <i>
#Option "Rotate" # [<str>]
#Option "TexturedVideo" # [<bool>]
#Option "Crtc2Half" # [<bool>]
#Option "Crtc2Ram" # <i>
Option "Int10" "True" # [<bool>]
#Option "AGPMode" "0" # <i>
#Option "DigitalScreen" "False" # [<bool>]
#Option "TV" # [<bool>]
#Option "TVStandard" # [<str>]
#Option "CableType" # [<str>]
#Option "NoHal" # [<bool>]
#Option "SwappedHead" # [<bool>]
#Option "DRI" # [<bool>]
Identifier "Card1"
Driver "mga"
VendorName "Matrox"
BoardName "MGA G400 AGP"
BusID "PCI:2:0:0"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Depth 8
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 32
Modes "1280x1024"
EndSubSection
EndSection

Section "Screen"
Identifier "Screen1"
Device "Card1"
Monitor "Monitor1"
DefaultDepth 24
SubSection "Display"
Depth 8
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024"
EndSubSection
SubSection "Display"
Depth 32
Modes "1280x1024"
EndSubSection
EndSection

:?

You don't appear to have any mode lines
Here is my XF86Config from suse8.2 using matrox dual head card 450

Section "Modes"
Identifier "Modes[1]"
Modeline "1280x1024" 114.82 1280 1280 1536 1736 1024 1072 1082 1166
Modeline "1024x768" 61.19 1024 1040 1216 1400 768 768 775 802
Modeline "1024x768" 71.39 1024 1040 1216 1400 768 768 778 802
Modeline "1024x768" 81.59 1024 1040 1216 1400 768 768 778 802
Modeline "1024x768" 91.63 1024 1040 1216 1400 768 768 778 802
Modeline "800x600" 37.44 800 816 928 1072 600 600 605 626
Modeline "800x600" 43.68 800 816 928 1072 600 600 610 626
Modeline "800x600" 49.92 800 816 928 1072 600 600 610 626
Modeline "800x600" 56.16 800 816 928 1072 600 600 610 626
Modeline "800x600" 62.40 800 816 928 1112 600 600 610 626
Modeline "800x600" 68.64 800 816 928 1112 600 600 610 626
Modeline "640x480" 23.96 640 656 720 864 480 480 484 501
Modeline "640x480" 27.96 640 656 720 864 480 480 490 501
Modeline "640x480" 31.95 640 656 720 864 480 480 490 501
Modeline "640x480" 35.94 640 656 720 864 480 480 490 501
Modeline "640x480" 39.94 640 656 720 864 480 480 490 501
Modeline "640x480" 43.93 640 656 720 864 480 480 490 501
EndSection

Section "Modes"
Identifier "Modes[0]"
Modeline "1280x1024" 102.24 1280 1296 1552 1664 1024 1024 1034 1070
Modeline "1024x768" 61.19 1024 1040 1216 1328 768 768 775 802
Modeline "1024x768" 74.45 1024 1040 1216 1328 768 768 776 802
Modeline "1024x768" 84.65 1024 1040 1216 1328 768 768 778 802
Modeline "800x600" 37.44 800 816 928 1040 600 600 605 626
Modeline "800x600" 45.55 800 816 928 1040 600 600 606 626
Modeline "800x600" 51.79 800 816 928 1040 600 600 607 626
Modeline "800x600" 58.03 800 816 928 1040 600 600 608 626
Modeline "800x600" 64.27 800 816 928 1040 600 600 609 626
Modeline "640x480" 23.96 640 656 720 832 480 480 484 501
Modeline "640x480" 29.15 640 656 720 832 480 480 485 501
Modeline "640x480" 33.15 640 656 720 832 480 480 486 501
Modeline "640x480" 37.14 640 656 720 832 480 480 486 501
Modeline "640x480" 41.13 640 656 720 832 480 480 487 501
Modeline "640x480" 45.13 640 656 720 832 480 480 488 501
EndSection

Thx marking,..I added the modes also,..and it still didnt work until I installed newer drivers,..now it works like a champ,..with all the modes too!!! All that time hacking at the config and it turned out to be a simple driver issue,..;( Atleast it works now...

I'm trying to get it to work on Fedora Core 2, and it's all kinda messed up too. Which driver were you using? The driver and powerdesk utility from the Matrox page didn't really work either.