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<?php $args=array(
    'post_type' => 'referentie',
    'posts_per_page' => -1,
    'orderby' => 'menu_order',
    'order' => 'ASC' 
);

$counter = 0;
$query = new WP_Query( $args );

if( $query->have_posts() ) : while( $query->have_posts() ) : $query->the_post(); $counter++;

    //We are in loop so we can check if counter is odd or even
    if( $counter % 2 == 0 ) : //It's even ?>
        <h1>1</h1>
   <?php else: //It's odd ?>
        <h1>2</h1>
   <?php endif;

endwhile; wp_reset_postdata(); endif;
?>