My favorite window manager is currently fluxbox but I recently wanted to try out wmii. So I searched for a way to run multiple window managers and still be able to switch between them. This is were Xnest comes in. With Xnest it is possible to - well - nest a X server.
We have to pass another display number to Xnest because the default
is normally already in use.
Xnest -ac :1
Afterwards we can start any application on our nested X server. The easiest way is probably to launch a terminal and issue further commands from there.
xterm -display :1
Note that within our new xterm the display environment variable is set accordingly:
echo $DISPLAY :1.0
Another way would be to simply set the display variable to 1 instead of 0 in the terminal where you started Xnest. All application will redirect their graphical output to the nested X server.
Next step would probably be to start a window manager in my case I just
wmii in my xterm and everything worked as expected.