Everyone who’s subscribed to the same communities will see all of each others’ comments. The ones that won’t be seen are those in communities a user intentionally doesn’t subscribe to, which is a good thing.
And putting the choice of where conversation takes place in the hands of the OP isn’t good. There’s already issues with the first poster in a “no duplicate submissions on the same topic” community getting to set the tone for conversation through title and text. This just makes it worse. Downvoting a bad link still means the conversation is being denied in the community of users’ choice and the solution to that is allowing duplicates, which is just the status quo plus extra spam.