I think a combined view is what people have been asking for, this sounds like a filter on the combined view of sub to me.
I guess it's a bit different if you're wanting to show comments as well. That sounds difficult to do in UX to make the context of a reply make sense, not sure how Mastodon does it, I guess there aren't super complex tree structures? Because the comment they could be replying to could have nothing to do with the original post, you have to show the entire tree some times