This complicated post is proposed as a complement to the item “mirroring” in the FAQ, in case it helps… Don’t hesitate to comment, I’ll adjust the unclear parts if needed 
The current FAQ entry on mirroring says 'This translates to a cluster setup, which is considered a (paid) enterprise solution by Nextcloud…’
But potentially, some of the objectives can be reached with the normal NextCloud instance, through a folder sharing between two NextBoxes (or a NextBox and another Nextcloud, if you trust its host).
You get indeed duplicated files in that folder, eliminating the risk of a single failure : it is definitely a mirrored folder, which works now, on the standard NextBox.
→ Note that what is not mirrored is all the rest : not your agenda, mail and generally any other app, nor your specific NextCloud details like user credentials, settings, and all server specific settings. It is not at all a full backup.
If you are still interested by (just) this mirrored folder of files, it’s quite fast to set up. Basically, I used the detailed how-to depicted here, but in a nutshell
-
you need to check ‘federation’ is present and activated (‘files’ left drop-down menu) -this was already set in my NextBox defaults
-
then, in the right dropdown menu parametres > administration > sharing, check
Allow Users On This Server To Send Shares To Other Servers
and
Allow Users On This Server To Receive Shares From Other Servers (again, both were enabled by default for me on the NextBox) -
then I checked Add Server Automatically Once A Federated Share Was Created Successfully which was not checked by default here
-
→ you need to do this setting up TWICE : on each one of your servers. Consider preparing two tabs in your browser, so that you can switch easily between them
-
You then get back to the ‘ordinary’ Files panel, create (or select) the folder you want to share, click on the ‘sharing’ tab and enter there the details for your other NextBox (or NextCloud server), in the form USERNAME@nameofyoursecondserver.dedyn.io (if you access it via dedyn as proposed here)
-
Then switch to your second server and ‘accept’ the sharing (a dialog will appear offering it)
-
You are done. You may rename the folders, so that in NextBox A you have 'folder shared with Nextbox B’ and on NextBox B ‘folder shared with Nextbox A’, for instance. For me at least it is clearer. (renaming will not cancel the sharing)
-
Check you can add a file in one of them : it appears in the other.
Note that if you delete the file on A and empty the trash, though, its B copy will only be put into B trash, but the B trash is not emptied until you ask so on B. -
You can ‘star’ these mirrored folders if you want, so that your Files panel shows them at the top. For me, basically, the Files panels now shows the shared folder first, and then all the rest that is just local.
