I'm actually curious about how you got the XGL part installed (I've been trying to get CentOS 5 + fglrx + XGL + beryl + dualhead as well). I got everything but XGL and beryl working.
As for the dualhead part. You want to not use XINERAMA or TwinView (nvidia specific) but instead use Pairmodes (ati specific). It basically amounts to having only 1 device and screen section (ie. you configure everything for one single screen) and then you add to the main Device section
Option "Mode2" "1920x1200,1600x1200" (second monitor modes) Option "PairModes" "1440x900+1920x1200,1440x900+1600x1200"
Once X is booted you can use xrandr (command line) to switch between various display modes (and aticonfig --lsp, aticonfig --set-powerstate=X, aticonfig --query-monitor also come in handy).
Here's my /etc/X11/xorg.conf (this is for an ATI fglrx driven 1440x900 LCD with occasional external monitor/LCD/projector):
Section "ServerLayout" Identifier "Layout" Screen 0 "Screen0" 0 0 InputDevice "Keyboard0" "CoreKeyboard" Option "Clone" "off" Option "Xinerama" "off" EndSection
Section "Files" EndSection
Section "Module" EndSection
Section "ServerFlags" Option "AIGLX" "false" Option "Xinerama" "false" EndSection
Section "InputDevice" Identifier "Keyboard0" Driver "kbd" Option "XkbModel" "pc105" Option "XkbLayout" "pl" EndSection
Section "Monitor" Identifier "Monitor0" Option "VendorName" "ATI Proprietary Driver" Option "ModelName" "Generic Autodetecting Monitor" Option "DPMS" "true" EndSection
Section "Device" Identifier "Device0" Driver "fglrx" Option "DesktopSetup" "horizontal" Option "Mode2" "1920x1200,1600x1200,1440x900,1152x864,1024x768,800x600,640x480" Option "PairModes" "1440x900+1920x1200,1440x900+1600x1200,1440x900+1440x900,1440x900+1152x864,1440x900+1024x768,1440x900+800x600,1440x900+640x480" Option "EnableMonitor" "lvds"
Option "MonitorLayout" "lvds,auto" Option "Centermode" "on" Option "DRI" "true" Option "VideoOverlay" "on" Option "OpenGLOverlay" "off" Option "EnablePrivateBackZ" "yes" BusID "PCI:1:0:0" EndSection
Section "Screen" Identifier "Screen0" Device "Device0" Monitor "Monitor0" DefaultDepth 24 SubSection "Display" Viewport 0 0 Virtual 1920 1200 Depth 24 Modes "1440x900" "1152x864" "1024x768" "800x600" EndSubSection EndSection
Section "DRI" Group 0 Mode 0666 EndSection
Section "Extensions" Option "Composite" "false" EndSection
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so any help on XGL and beryl?
Thanks, Maciej