As the first step in more collaborative thoughtstreaming, we've added combined streams. The URL /combined/<slug>/ will now show a stream that combines all (published) streams with that slug, regardless of user.
For example: if you want to extend my New TL;DRs post, just create a stream with the same name (or at least the same slug) and it will be included in https://thoughtstreams.io/combined/the-new-tldrs/
This combined stream is currently not actionable (you can't repost from it, etc) but that will come soon as well as a more controlled form of collaborative thoughtstreaming where you'll be able to set up "group" streams.