login screen with dual monitors in ubuntu 17.10

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.