I haven't really looked close, but these 2 files may be what you need to pursue farther to accomplish what you need.
Those 2 seem to be what it looks at when it decides if the user is authorized to shutdown/reboot the system.
There is probably a way to add a paragraph to the /etc/dbus-1/system.d/org.freedesktop.login1.conf to tell it that a useris authorized to shutdown/reboot with multiple users, but I haven't looked at it much.
Just a warning if you go mucking about with these files...
Back up the originals, and have a live boot cd handy that you can boot from and restore the original files.
You could easily render your system unusable if you mess these files up.