Link/Sync 2 Nextcloud servers to share user names/pwd/calendar and contacts only

Hi all,

I am running 2 instances of Nextcloud @ Home.

  1. Nextcloud-A: On my NAS - this one holds all our Media, and scans from important documents (Legal). This Nextcloud instance cannot be accessed from the Internet).
  2. Nextcloud-B: On my Server (Mail/Web/DNS). This one can be access from the Internet

So - my Internet server is supposed to sync the pictures taken from our phones (all family), calendars, contacts.
Currently I use openvpn to connect to the NAS on our LAN - but for some elderly members of the Family, the concept of different networks is quite an issue.

My idea is to have a internet facing Nextcloud instance for our Social stuff, and a LAN secluded entity for important documents etc. - On top of that - as the LAN instance has way more storage, use that one to secure/backup the data from our Internet facing Nextcloud instance.

What is the best way for doing this? Any hints? Can I link Nextcloud-A to Nextcloud-B without having to manually replicate the users?

FYI - I build my own Docker images for nextcloud - current build uses:
Nextcloud version: 17.0.5
Operating system and version: Alpine 3.8 + Updates
Apache or nginx version: NGinx 1.17.9
PHP version: 7.2.29
HA-Proxy: 1.9.13
Redis: 5.0.6

Anyone? I would not want to use an external way of doing that, or directly perform a mysql sync on calendar and contact stuff.

Maybe using nextcloud federation would solve this. I have not had a chance to play with this yet though so it may be a dead end.

Would be interested to hear what you find out.

already tried. No option for that in the federation. it is a trust system apparently, but nothing more.