I have two custom post types, one of which I need to be able to select as a parent of the other. Post type A has a url structure www.mydomain/posttypeA/posttypeAsingle, and I need to allow post type B to select a parent from post type A so the url displays www.mydomain/posttypeA/posttypeAsingle/posttypeB

By creating Post Type B as such, it allows me to select a parent of ITS OWN post type, but does not allow me to select a parent from a different post type (Post Type A):

register_post_type('posttypeB',
        array(
            'labels'      => array(
                'name'          => __('Post Type B', 'textdomain'),
                'singular_name' => __('Post Type B', 'textdomain'),
                'add_new'       => __('Add New Post Type B'),
                'add_new_item'       => __( 'Add New Post Type B' ),
                'edit_item'          => __( 'Edit Post Type B' ),
                'new_item'           => __( 'New Post Type B' ),
                'all_items'          => __( 'All Post Type B' ),
                'view_item'          => __( 'View Post Type B ),
                'search_items'       => __( 'Search Post Type B' ),
                'not_found'          => __( 'Nothing found' ),
                'not_found_in_trash' => __( 'Nothing found in the Trash' ), 
                'parent_item_colon'  => '',
                'menu_name'          => 'Post Type B'
            ),
                'public'      => true,
                'description'   => 'Sessions are posted here',
                'public'        => true,
                'menu_position' => 4,
                'hierarchical'     => true,
                'supports'      => array( 'title', 'editor', 'thumbnail', 'excerpt', 'comments', 'page-attributes' ),
                'taxonomies'    => array('category', 'post_tag'),
                'menu_icon'     => 'dashicons-welcome-learn-more'
        )
    );

How can I allow posts belonging to Post Type B, to select a parent from Post Type A in order to maintain the desired URL structure?