http://wiki.archlinux.org/index.php/X#Virtual_X_session

Virtual X session

To start another X session in, for example, CTRL + ALT + F8, you need to type this on a console:

 xinit /path/to/wm -- :1

Change "/path/to/wm" to your window manager start file or to your login manager like gdm, kdm, or slim.

http://wiki.archlinux.org/index.php/X#Nested_X_session

Nested session

(of another desktop environment:)

$ /usr/bin/Xnest :1 -geometry 1280x768+0+0 -ac -name name of session & wmaker -display :1

This will launch another session in a 1024 by 768 window within your current X session.