Login managers such as KDM and GDM include a "switch user" function which facilites switching users (in the GUI DE or WM), and allows easy login of new users.

Suppose one wants a more streamlined environment---eg something like Fluxbox or OpenBox running with SLIM, XDM, or no login manager at all----

Once multiple users are logged in, you can always switch them with ctrl-alt-F7, F8, F9, etc.

One good solution---at least in my case--would be a mechanism to automatically log in several users such that they always are assigned to the same VT---eg my account would always be accessible with ctrl-alt-F7, and my wife would always be on F8.

If all else fails you can easily just edit inittab to spawn X sessions directly as opposed to prefdm.