Nautilus removed extra pane functionality near version 3.6 (so Ubuntu 12.04 LTS was the last one with F3extra pane) - we have bug 1173648 from 2013-04-27, 35 users are affected about this.
You can drop Nautilus, which lacks numerous useful features including this one and doesn't look like it will be getting them anytime soon, and use Caja from MATE DE.
There's no other way to get the split window feature you're looking for while still using a file browser that looks and works like Nautilus.
Caja is the official file manager for the MATE desktop. It allows for browsing directories, as well as previewing files and launching applications associated with them. It is also responsible for handling the icons on the MATE desktop. It works on local and remote filesystems. Caja is a fork of Nautilus.
Pressing F3 or selecting View -> Extra Pane will do what you want.
You can install it with
sudo apt-get install caja
You can try to select Caja as default file-manager with
exo-preferred-applications (installed by
sudo apt-get install exo-utils) or with
mate-default-applications-properties (installed by
sudo apt-get install mate-control-center). But I did not get success here on standard Ubuntu 16.04 LTS Unity session, not sure about 17.10.
So the best way to use Caja is to use it in its native environment - MATE DE (installable with
sudo apt-get install ubuntu-mate-desktop).