Query Multiple Post Types with same Query

gistfile1.php

//try to include two post types in main query
function query_multiple_posts_types( $query ) {
    if ( is_home() ) {
        //Display 20 posts
        $query->query_vars['posts_per_page'] = 20;
        //Display multiple post types
	$query->query_vars['post_type'] = array('post', 'portfolio');
        return;
    }
}
add_action('pre_get_posts', 'query_multiple_posts_types', 1);