Could it be possible to share a thread with someone? Having literally two posters for the same thread, both gaining +1 in postcount and having the same permissions to the post.
If that's not possible, then maybe an "invite additional editor" kind of function. Say you created a thread that you want to keep updated, but you won't be around too much, and there aren't any moderators who can help you either. THen you could invite your friend to be an editor of that thread, which would simply give your friend permission to edit your thread without being a moderator.