WordPress Co-Authors-Plus: template helpers for getting co-authors for feeds. You must specify the dublic core namespace in your xml document.

cap_feed.php

<?php

function coauthors_feed() {
	$authors = get_coauthors_feed(); // array of creator nodes, one for each author
	echo wp_sprintf( '%l', $authors );
}

// uses dublin core specification for creator, so please specify dc namespace.
function get_coauthors_feed(){
	$output = array();
	$authors = get_coauthors();	
	foreach ( $authors as $author ) {
		$output[] = sprintf( '<dc:creator><![CDATA[%s]]></dc:creator>',  $author);
	}
	return $output;
}