There is a "Switch User" button on the logout menu.
However! This does lock the current session on tty8. You can switch between them using Ctl-Alt-F8 and Ctl-Alt-F9 but have to re-enter the password.
Another way is via the command gdm-control --switch-user
which acts immediately and doesn't lock the current session. Again, use Ctl-Alt-F8/9/10/11/... to switch between active sessions.
I use this latter method very frequently. There is an issue with the session backgrounds getting blitted if one continually swaps back and forth (which I've never raised because it's IMO in the "who cares" region).