The two messages serve two distinct purposes:

MediaWiki:Commentbox-first-comment-heading is the current heading, that should be added to the page, when no comment heading is present on the page.

MediaWiki:Commentbox-regex is a regular expression matching the current heading *and any historic heading*. That way, if Commentbox-first-comment-heading is changed, the regex should match both the old header and the new header.

This feature is actually in use on the wiki I first developed this extension for (I later added an image to the comment heading), so removing the second message would break this site and would therefore be a regression.

That being said, I understand that it would be easier to only have one message to translate. I don't currently have time to work on the extension, but I would accept a patch that changes MediaWiki:Commentbox-regex from a MediaWiki message to a config variable. It could be set in LocalSettings.php. If it was not set there, the extension would generate the regex itself.