Set up Xinerama
We use Xdmx to create a virtual Xinerama display on top of our two nested heads. We'll use :3 as the display number.
The basic invocation is as follows but see the notes below.
Xdmx +xinerama -xinput local -display :1 -display :2 -ac :3 &
On my nVidia system no windows were visible unless I added -noglxproxy.
On Ubuntu I had to use the Xdmx binary from Fedora Core 6. As Ubuntu X11 is configured to use a different font path I had to explicitly tell Xdmx about it. Adding -fontpath `xset q | sed '1,/^Font Path:/d;q'` did that.
Also on Ubuntu - and probably also to do with compile-time differences between the two distributions - I had to disable the XKB extension with -noxkb or I was unable to type anything in my Xinerama display.