Co-authors plus problems with query_post

Please don’t use query_posts. Just don’t. Create a new query with WP_Query.

Secondly, I am not sure how you are trying to use multiple authors in the query but I assume you are using author and a comma separated string of IDs or author__in and an array. For example:

$query = new WP_Query( 'author=2,6,17,38' );

// or
$query = new WP_Query( array( 'author__in' => array( 2, 6 ) ) );

http://codex.wordpress.org/Class_Reference/WP_Query#Author_Parameters

Either should work.

Also:
showposts has been deprecated for a long time now. You should be using posts_per_page. I also suspect that you should be using a filter on pre_get_posts rather than creating a new query at all.