If you use Ubuntu 17.10 with dual monitors you’ll notice that the login screen won’t keep the same settings as your desktop. The workaround goes like this:
Login and set up the displays as you please, using the settings control panel. Then, run the following commands on a terminal:
sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config/ sudo chown gdm.gdm /var/lib/gdm3/.config/monitors.xml
And reboot. You’ll have your login screen with the same settings as your desktop.