The Switch User action has been added to the logout dialog super + x .
It allows you to run two (or more?) X sessions for different users and switch between them.
If you already have two sessions running, you can switch between them much faster using keyboard shortcuts:
Ctrl + Alt + F7 - switch to first session
Ctrl + Alt + F8 - switch to second session
This feature was requested by some user few months ago, unfortunately I can’t find where.