I don't know how the code is currently working, but I like this feature idea and would suggest to start very simple and proceed from there.

For example you could: a) Make a list of communities that are siblings with their id and instance b) add a toggle to view sister community posts yes /no c) query all communities, list the last x posts from each with time constraints, e.g. not older than 1 day or hour depending on the community post frequency d) list them sorted by time of x , depending on what was chosen

The biggest issue I see with this simple approach, besides others, is that different communities are different in terms of activity / post frequency. So ideally the better, but more effort, way would be to let each community instance communicate their posts themselves via a query with activity metric parameters. Basically the amount of returned posts would depend on common parameters set by the most active instance.

It's not yet thought out, but just getting an mvp started and test the waters would probably be better than having it perfect right away while working on it for months