Another 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.
(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.