(Sorry if this seems messy! I'm trying my best.)
I had an idea of replying to multiple people, after seeing a post where the creator had to reply the same thing twice to notify and tell two people who asked the same question.

This would work similarly to emails; you can set it to reply to just the person you clicked "Reply" on, or type another address in so that multiple people can be notified. Unlike emails, however, these would be visible to whoever saw the post.
This could be a "replying to:" box above the normal text box, which you can write @mentions in to specifically reply to certain people. (I am unable to edit anything in except for a few lines on a screenshot, as I don't really know how to do more lol)

On wall & guest book posts, it could say something like: "(Replier) replied to (commenter)'s comment & (commenter)'s comment below (how long ago it was)" above the reply in question.
Meanwhile, forum and submission comments could appear like this:

It's a bit difficult to understand, so it would look much better on the actual site.
Here is the link to the post I got the screenshot from. I would link the specific comment these are replying to, but I can't, as there's no button to view it alone.
(it was a poor choice of multiple replies, and I'm not saying that Olonea was replying to both, I was just looking for a place where I could find replies in that order)