Emulating extra displays
Solution 1:
I found the solution using Xephyr and Xdmx
First create the Xephyr displays
Xephyr -screen 1920x1080 -ac :1
Xephyr -screen 1920x1080 -ac :2
Xephyr -screen 1920x1080 -ac :3
This creates 3 windows 1920x1080, each of these will act as a monitor
Next add the xinerama layer onto these virtual displays
Xdmx +xinerama -xinput local -display :1 -display :2 -display :3 -ac :4
Next create a fresh terminal and start your window manager or application like so
DISPLAY=:4.0
xmonad
or if you just wanted to test an applications xinerama support instead of xmonad
you could run xterm
or thunar
.
Solution 2:
You only need a single instance of Xephyr.
Xephyr +xinerama -screen 1920x1080 -screen 1920x1080 -screen 1920x1080 -ac :1
If you need RANDR extension, add +extension RANDR
args.
Xephyr +extension RANDR +xinerama -screen 1920x1080 -screen 1920x1080 -screen 1920x1080 -ac :1