Bridging communities

I worked on a project like this many years ago, but was mainly using lesser used features that aren't available anymore unfortunately as far as I remember.

I would think it would be possible to do, but would be custom development.

Search

You'd want to point both the communities at the same search index (either SOLR or some other provider). The customisations would need to have some extra information added so that you can identify which community the content is from, you should be able to do this wth a custom ISearchProvider adding in the extra details in the IndexContent method. You'd then need to update where the results are rendered to make sure the links go to the correct site.

Registration

Think there are two options, the one you described where API calls are made to keep both communities updated. The other would be to have a central login and profile system seperate from the community where changes are made and then update the communities from there. Whichever approach need to be careful of having a source of "truth" for the user and making sure data updates are replicated correctly across all copies (even if something goes wrong).

Legacy Users

Yep this will depend a lot on the current state of your communities - do they both exist already, or is one going to be new? If they already exist is it possible users have accounts in both with different usernames or emails, in which case will need a way to merge those accounts. If its a new second community this may be easier. Reporting and knowing which users are "members" of each Community is also one that depends on the approach a little, and maybe on local legal requirements (e.g. is it ok to automatically create a users account in a seperate community without the user signing up to it). The easiest option would probably be to only create a member in each communities database when the user goes to interact with that specifiic community, you may need to make some customisations to the login/registration flow to make this clearer to users and easier to do so they know they can share the same details.

Hope this helps,
Rhys

Thank you Rhys! This is great information. I really appreciate the thorough response.

Are you able to share more specifics on any specific use cases driving the need (ex. Internal support team accessing public requests) and expected user experiences for the community members? For example, are users within each community aware of both communities and its members or is this for a subset of users? Are search results from both communities relevant and obvious to users across both communities; i.e. if I joined community A but see results from community B is it clear why I see those?