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<?php

$html = '';

// Get all Child Pages
$args_1 = array(
    'post_status' 	=> 'publish',
    'post_type'		=> 'custom_post_type_1',
    'orderby' 		=> 'title',
    'order'     	=> 'ASC',
    'suppress_filters' => false,
);

$query_1 = new WP_Query( $args_1 );
$posts_1 = $query_1->posts;

$args_2 = array(
    'post_status' 	=> 'publish',
    'post_type'		=> 'custom_post_type_2',
    'orderby' 		=> 'title',
    'order'     	=> 'ASC',
    'suppress_filters' => false,
);

$query_2 = new WP_Query( $args_2 );
$posts_2 = $query_2->posts;

$_posts_merged = array_merge( $posts_1, $posts_2 );
$_posts_ids = array();

foreach ( $_posts as $key => $value ) {
    array_push( $_posts_ids, $value->ID );
}

$comma_separated_post_ids = implode(',', $_posts_ids); // We do this because of WPML

$args_list_pages = array(
    'echo'		=> false,
    'include'	=> $comma_separated_post_ids,
    'title_li'	=> '',
    'suppress_filters' => false,
);

$html .= '<ul>';
$html .= wp_list_pages( $args_list_pages );
$html .= '</ul>';