Alternately, it might be interesting to just send a notification to the respondee alerting them to the presence of the responder's new stream, and not add it by default. The link from the responder's card to the respondee's stream would be automatic, but the need for action on the part of the respondee in order to hook up the discussion would allow for a more curated discussion and would allow heated exchanges to proceed more deliberately than the usual commenter pile-on.