Option "XAANoOffscreenPixmaps" "1"
a la Section "Device" para cualquiera de las 2 placas en el Xorg.conf.
Por las dudas, por si a otros les sirve, pego aqui mi actual Section "Device" para la Ati Radeon 9600 que estoy usando en este momento.
Section "Device"
### Available Driver options are:-
### Values: : integer, : float, : "True"/"False",
### : "String", : " Hz/kHz/MHz"
### [arg]: arg optional
# la sgte linea me arregla el problema de la Geforce FX5200 con las imagenes de facebook
# curiosamente la Ati Radeon 9600 me hace el mismo problema y la sgte opcion tambien me la arregla
Option "XAANoOffscreenPixmaps" "1"
Option "NoAccel" "False" #Option "NoAccel" # []
#Option "SWcursor" # []
#Option "Dac6Bit" # []
#Option "Dac8Bit" # []
#Option "BusType" # []
#Option "CPPIOMode" # []
#Option "CPusecTimeout" #
#Option "AGPMode" #
#Option "AGPFastWrite" # []
#Option "AGPSize" #
#Option "GARTSize" #
#Option "RingSize" #
#Option "BufferSize" #
#Option "EnableDepthMoves" # []
#Option "EnablePageFlip" # []
#Option "NoBackBuffer" # []
#Option "DMAForXv" # []
#Option "FBTexPercent" #
#Option "DepthBits" #
#Option "PCIAPERSize" #
#Option "AccelDFS" # []
#Option "DDCMode" # []
#Option "IgnoreEDID" # []
#Option "DisplayPriority" # []
#Option "PanelSize" # []
#Option "ForceMinDotClock" #
#Option "ColorTiling" # []
#Option "VideoKey" #
#Option "RageTheatreCrystal" #
#Option "RageTheatreTunerPort" #
#Option "RageTheatreCompositePort" #
#Option "RageTheatreSVideoPort" #
#Option "TunerType" #
#Option "RageTheatreMicrocPath" #
#Option "RageTheatreMicrocType" #
#Option "ScalerWidth" #
#Option "RenderAccel" # []
#Option "SubPixelOrder" # []
#Option "ShowCache" # []
#Option "DynamicClocks" # []
#Option "VGAAccess" # []
#Option "ReverseDDC" # []
#Option "LVDSProbePLL" # []
#Option "AccelMethod" #
#Option "DRI" # []
#Option "ConnectorTable" #
#Option "DefaultConnectorTable" # []
#Option "DefaultTMDSPLL" # []
#Option "TVDACLoadDetect" # []
#Option "ForceTVOut" # []
#Option "TVStandard" #
#Option "IgnoreLidStatus" # []
Identifier "Card0"
Driver "radeon"
VendorName "ATI Technologies Inc"
BoardName "RV280 [Radeon 9200 PRO]"
BusID "PCI:1:0:0"
EndSection