Hi,
Your bug is quite old, sorry for the delay. Could you please provide your /etc/X11/xorg.conf ? You should have something like that in it, with the size of both screen in Virtual :
SubSection "Display"
Viewport 0 0
Virtual 2800 2800
Depth 24
EndSubSection
If this is not or badly defined, you will probably not be able to use the full resolution of your screens.
Regards,
Adrien