How can I duplicate my desktop and show it side-by-side on a single screen?

Most of what I am writing will be coming from this SU question, or more specifically this answer.

Since you are using raspbian, you're going to have to use FakeXRandr, which has FakeXinerama built-in:

Download FakeXRandr from this link. Unzip the archive,cd ~/Downloads/fakexrandr-master and run

sudo make
sudo make install
sudo apt-get install xorg-dev libglu1-mesa-dev build-essential python3-gi python-gi-dev python-gi-cairo

You will now have the tool fakexrandr-manage available to use. Launch that from terminal, and you will see the following GUI to configure your monitor setup:

One post suggest this (Refer this link: (virtual) split screen on Ubuntu)

As far as I know you can only split your screen for two windows at a time. Either one on the left and one on the right, Or one on the top and one the bottom. And there are shortcuts associated for these actions which are:

For vertical split:

Ctrl + Win + ←→

For horizontal split:

Ctrl + Win + ↑↓

In in case this does not work As You have not mentioned you are using Ubuntu 16.04 or 18.04. You can refer this post "(virtual) split screen on Ubuntu" & "https://superuser.com/questions/115076/how-to-vertically-split-widescreen-into-two-virtual-workspaces-on-ubuntu-gnome" Here FakeXRandR is suggested for 18.04.