To display posts from either tag you need to use tag__in instead of tag__and . This will return posts from either tag id.
$tag_query = new WP_Query( array( 'tag__in' => '13, 27' ) );
To display posts from either tag you need to use tag__in instead of tag__and . This will return posts from either tag id.
$tag_query = new WP_Query( array( 'tag__in' => '13, 27' ) );