I've noticed that it's not possible to query for categories and authors at the same time.
When searching for a category, the generated SQL makes INNER JOINs on the "term_relationships" and the "term_taxonomy" tables twice (once for the category and once for the authors). This doesn't work!
I've made a slight change to the SQL in the plugin to use aliases on the joins which fixes the problem.
I've changed line 171 in co-authors.php:
$join .= " INNER JOIN $wpdb->term_relationships AS coauthor_relationships ON ($wpdb->posts.ID = coauthor_relationships.object_id) INNER JOIN $wpdb->term_taxonomy AS coauthor_taxonomy ON (coauthor_relationships.term_taxonomy_id = coauthor_taxonomy.term_taxonomy_id)";
and then line 186:
$where = preg_replace('/(\b(?:' . $wpdb->posts . '\.)?post_author\s*=\s*(\d+))/', '($1 OR (coauthor_taxonomy.taxonomy = \''. $this->coauthor_taxonomy.'\' AND coauthor_taxonomy.term_id = \''. $term->term_id .'\'))', $where, 1); #' . $wpdb->postmeta . '.meta_id IS NOT NULL AND
Hope that helps if anyone else was having the same problem.