Comment RSS mixed with post RSS

I’m looking for a plugin or “simple” solution to combine both the standard post RSS feed with the comment RSS feed. So if either get updated, there is a new entry in this combined feed.

I know I could use pipes or some external service, but that seems silly when wordpress is obviously generating both rss feeds internally.

In spite of several that seemed “promising”, I can’t seem to find a plugin that does this. Anyone know of one?