Newbie needs help for clean start / taking over the previous dev' :)

Hi guys!

I am totally new to wordpress (usually working on Squarespace) and my question is the following: I need to link 2 wordpress sites in such a way so that they have the same design, although the both domain names can stay as they are:

https://close-combat.fr/
https://close-combat.org/

I didn’t work on those sites but the first one is quiet old and in need of an update.
Can someone give me a quick hand to go through these first steps before i start?
I do not fully grasp yet the difference between wordpress.com and .org
Any help is more than welcome!
(Also i am not sure about the exact wordpress version of the sites)..

Thanks a lot!

Cheers ??