Just installed a fresh copy of Maverick, and went from my two monitors working perfectly to broken in seconds.
As usual I configured the monitors in the display applet, this worked fine. I also hit Make Default which gave me no problems. I then restarted and the monitors had gone back to Mirror Screen. I changed them back again and hit Make Default which gave me a message about setting the Virtual Desktop resolution. After this any changes I make to the monitors give me the message "Please log out and back in again" - I've tried that and restarting but still cannot configure the monitor resolutions.
Any ideas on how I can revert this? Some searching indicates the problem's with xorg but I've no idea where to go further than this.
Here's the relevant
01:00.0 VGA compatible controller: ATI Technologies Inc M92 [Mobility Radeon HD 4500 Series]
I tried extending the desktop using ATI's Catalyst tool but this still mirrored the screens. Does this mean the drivers aren't being used?