List latest post out of 2 custom post types

Just set post_type to an array of types, and posts_per_page to 1 and you’ll get the most recent post from those types.

$args = array(
    'post_type' => array( 'type_1', 'type_2' ),
    'posts_per_page' => 1
);
$latest = new WP_Query( $args );