Multisite - One Site with multiple URLs

Short answer is - this is possible and easily achievable by using JJJ's plugin https://wordpress.org/plugins/wp-multi-network/.

Long answer is - WordPress allows for such structures to be created, but does require some hacking in order to make it right if you decide to do it manually. The original database structure of a multisite website is prepared for meeting such requirements, but the administrator interface lacks the controls. My guess is that it was limited in order to avoid what could be a great mess done by less experienced users, as multisite by itself is a complex concept and quite often misused.

Here's an article that explains how you can use the forementioned plugin, but also shows where in the database you could check the results of your configuration, which could be a hint for you how to do the whole thing manually without a plugin: https://wpmudev.com/blog/wordpress-multi-network/

P.S. I would advise you to consider using multilingual plugins if you haven't done so already, as in plenty of the cases these could save you time and efforts when setting up a whole multisite network. Also beware that some services and plugins have limitation when it comes to multisite, so things like backing up or migrating the site might become slightly more complex (or costly), if you are used to plugins line Duplicator or services like Blogvault.