You need to add this code at the bottom of functions.php
function is_multi_author() {
global $wpdb;
$rows = (array) $wpdb->get_col("SELECT DISTINCT post_author FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish' LIMIT 2");
$is_multi_author = 1 < count( $rows ) ? 1 : 0;
return (bool) $is_multi_author;
}