This guide explains how to set up a fake Xinerama display on a single head. This allows developers to test how their application works on a Xinerama display even if they don't have a dual-head setup.
There are several ways to achieve this. The most reliable uses Xdmx. Unfortunately the latest versions of Xdmx as packaged by Fedora, Ubuntu and probably other distributions (and maybe even other operating systems) are known to crash out. I found that the Xdmx which ships with Fedora Core 6 works well. You can download it here:
In brief, the method is to first create two nested X displays then use Xdmx to join them into one dual-head Xinerama display.