Xfce4-settings > Display > Mirroring displays.

https://docs.xfce.org/xfce/xfce4-settings/display

It does even say in the current documentation that:

Mirror Displays
Display the same content on all monitors.

My problem or issue is the "all" part there. I have 4 displays, I only want 2 of them to show the same content, not all 4 of them. Can't do it right now.