I don't think having separate subforums would help that, but I could make a thread merge request link.
So someone could ask to merge the current thread with a different thread by id (the number after /post/ in the url).
Then maybe it would get reviewed by me to be sure it's a sane request, and if so, the original post on the later thread would just become a comment on the earlier thread. And all of the comments on both would get put into the earlier thread.
And if anyone is watching the later thread (getting comments by email) then that watch could be moved to the earlier thread. I suppose it would break bookmarks of the later thread though.
I have no idea. I only read a couple of posts of Reddit when they show up in a search page. And from your page, I read what shows up in my email. Sometimes I open a post when the title jumps out to me. I will go with what you think is best.
Tell me what it should look like if you think it's a good idea.