RSS for recently modified posts

In my index.php, I have the posts “newest first” which is what I want. But I edit old posts quite often, and so I have a list of “recently modified” posts in the sidebar. How do I create an RSS feed for the list of “recently modified” posts — separate from the main RSS for “new” posts?

Can I just copy the “wp-rss2.php” file and modify it somehow? Will other functions break if I do that?

Blog in question: Bookp(h)ile