Create nested displays
Here we make the assumption that :1 and :2 are available for use as nested displays.
We also assume that a 1440x900 Xinerama display is required, and so create two 720x900 "halves" - change the geometry as applicable.
The preferred method is to use Xephyr.
Xephyr -screen 720x900 -ac :1 & Xephyr -screen 720x900 -ac :2 &
The -ac option tells Xephyr to allow access to the display from anywhere. If you are security conscious you might want to turn omit that option. If you are security conscious you will know what else you need to do. Security is outside the scope of this article.
If you don't have or can't use Xephyr you could also try Xnest.
Xnest -geometry 720x900 -ac :1 & Xnest -geometry 720x900 -ac :2 &
Now we have two nested displays let's set up Xinerama on top of them.