One feed, two queries – problem with scheduled posts

I have an RSS feed configured to show the 20 most recently modified posts. However, I’m facing an issue where posts scheduled far in advance do not get included in the feed, as they do not appear as recently modified.

Is there a way to modify a query to order by modified/published date while also giving both the same weight?

Here’s what the code looks like currently.

$args = array(
	'orderby'=> 'modified',
	'order' => 'DESC',
	'showposts' => '20',
	'post_type' => 'post',
	'post_status' => 'publish'
);
$posts = query_posts($args);