LXDM supports user switching with lxdm -c USER_SWITCH and it seems that lightdm supports too simply by starting another instance as in the example for xscreensaver at https://wiki.archlinux.org/index.php/Xscreensaver#LXDM
Actually lightdm ships with gdmflexiserver too. https://wiki.archlinux.org/index.php/Lightdm#User_switching_under_xfce4 Here in Fedora 17, lightdm 1.2.2-17, it's in /usr/libexec/lightdm/gdmflexiserver
This project has been archived, so we are closing related bugs