Everyone who’s subscribed to the same communities will see all of each others’ comments.

This still relies on everyone using the same app/front-end.

I guess I’m thinking about how it would be helpful in more general cases. If someone has an issue with a FOSS app, and they ask about it in two small communities, it would be much better to have the troubleshooting discussion in one place rather than have both communities missing part of the context.

Ultimately in your example, the user can still make both posts, this doesn’t change that. It just directs the comments to one post’s comment section rather than having it spread out.

Still it’s good to think about. Would a good middle ground just be the first implementation then? To link to the post that OP intends to be the main discussion thread, but people are free to ignore that if they want.