You can 'su <user>' to change to a different user for the current tty, or if you want to have another entire X session running as a different user, get a new console:
Ctrl-Alt-F1 (for example)
Then, login as the new user and type,
startx -- :1
You can then swtich between X sessions with
Ctrl-Alt-F7 (typically the first X session) Ctrl-Alt-F8 (the new X session)
Note that the particular function keys will depend on the your tty configuration, but these are typical.
Guus. -- A.J. Bonnema, Leiden The Netherlands, user #328198 with the Linux Counter