From the bug-report there’s a simple solution to dual-screening on Hardy…

Users upgrading from Dapper Drake to Hardy Heron who have used Xinerama to support dual monitors now have to use a different method (xrandr I think). This isn’t mentioned in the release notes and the graphical tool for configuring dual monitors is under “Screen Resolution” in the System -> Preferences Menu. The point being that it is quite hard to find.

Users with multiple desktops appear to have add the following to the xorg.conf file “Screen” section:

SubSection “Display”
Depth 24
Virtual 4000 2048
EndSubSection

Then the screen resolution tool works fine for me.

Me too! Just to complete this (it’s all out there…):

Unfortunately, for creating dual-screen layouts there is still one manual configuration step required, which is to add a Virtual framebuffer size. The size needs to be equal or greater than the maximum combined size of your displays. For example, if you have two 1920×1200 monitors you wish to put side-by-side, you would add a Virtual line like this:

Section "Screen"
        Identifier      "Default Screen"
        Device          "Configured Video Device"
        DefaultDepth    24
        SubSection "Display"
            Depth           24
            Virtual         3840 1200
        EndSubSection
EndSection

Note that setting Virtual to larger than 2048×2048 disables 3d acceleration (i.e., no Compiz).