How to get tagged pages to show up with tagged posts?

You need to use pre_get_posts hook. Paste this code to your themes functions.php file.

add_action('pre_get_posts', 'wpse_pre_get_posts');
function wpse_pre_get_posts($q)
{
    if( $q->is_main_query() && $q->is_tag() )
    {
        $q->set('post_type', array('post','page') );
    }
}