A WordPress custom rewrite rule example, combining two post types.

matty-double-post-type-rewrite-rule-example.php

<?php
/**
 * Plugin Name: Double Post Type Rewrite Rule Example
 * Plugin URI: https://gist.github.com/mattyza/7dfd156992f23835f68e
 * Description: Adds a custom rewrite rule to mimic http://domain.com/post-type-01/post-type-02/.
 * Author: Matty Cohen
 * Author URI: http://matty.co.za/
 * Version: 1.0.0
 * Stable tag: 1.0.0
 * License: GPL v3 - http://www.gnu.org/licenses/old-licenses/gpl-3.0.html
 * Requires at least: 4.1.0
 * Tested up to: 4.1.0
 *
 * Text Domain: matty-double-post-type-rewrite-rule-example
 * Domain Path: /languages/
 */

new Matty_Double_Post_Type_Rewrite_Rule_Example();

class Matty_Double_Post_Type_Rewrite_Rule_Example {
	private $primary_post_type_token;
	private $secondary_post_type_token;

	public function __construct () {
		$this->primary_post_type_token = 'product';
		$this->secondary_post_type_token = 'page_slug';

 		add_action( 'generate_rewrite_rules', array( $this, 'create_rewrite_rules' ) );
 		add_filter( 'query_vars', array( $this, 'add_query_vars' ) );
 		add_action( 'template_include', array( $this, 'process_rewrite_request' ) );
	} // End __construct()

	/**
 	 * Generate our custom rewrite rules and add them to the array of registered rewrite rules within WordPress.
 	 *
 	 * @access public
 	 * @since  1.0.0
 	 * @return array
 	 */
 	public function create_rewrite_rules () {
 		global $wp_rewrite;

		$primary_post_type_token_tag = '%primary_post_type_token%';
		$secondary_post_type_token_tag = '%secondary_post_type_token%';

		$wp_rewrite->add_rewrite_tag( $primary_post_type_token_tag, '(.+?)', 'post_type=product&' . $this->primary_post_type_token . '=' );
		$wp_rewrite->add_rewrite_tag( $secondary_post_type_token_tag, '(.+?)', $this->secondary_post_type_token . '=' );

		$rewrite_keywords_structure = $wp_rewrite->root . "entry/$secondary_post_type_token_tag/$primary_post_type_token_tag/";
		$new_rule = $wp_rewrite->generate_rewrite_rules( $rewrite_keywords_structure );
		$wp_rewrite->rules = $new_rule + $wp_rewrite->rules;

// To view the rewrite rules array, uncomment the line below and visit the Settings > Permalinks screen.
// echo '<xmp>'; print_r( $wp_rewrite->rules ); echo '</xmp>'; // DEBUG

		return $wp_rewrite->rules;
 	} // End create_rewrite_rules()

 	/**
 	 * Register our custom querystring variables within WordPress.
 	 *
 	 * @access public
 	 * @since  1.0.0
 	 * @param  array $query_vars
 	 * @return array $query_vars
 	 */
 	public function add_query_vars ( $query_vars ) {
 		$query_vars[] = $this->primary_post_type_token;
 		$query_vars[] = $this->secondary_post_type_token;

 		return $query_vars;
 	} // End add_query_vars()

 	/**
 	 * Flush rewrite rules on init of WordPress. FOR DEVELOPMENT PURPOSES ONLY.
 	 *
 	 * @access public
 	 * @since  1.0.0
 	 * @return void
 	 */
 	public function flush_rewrite_rules () {
 		global $wp_rewrite;
 		$wp_rewrite->flush_rules();
 	} // End flush_rewrite_rules()

 	/**
 	 * Process our rewrite rules request.
 	 *
 	 * @access public
 	 * @since  1.0.0
 	 * @param string $action
 	 * @param string $type
 	 * @param string $key
 	 * @return array $response
 	 */
 	public function process_rewrite_request ( $tpl ) {
 		// To view the various query variables we've registered, uncomment the lines below and attempt to visit a URL using your custom rewrite rule.
 		// eg: http://domain.com/sample-page/flying-ninja/.

 		// echo '<xmp>'; print_r( get_query_var( 'product' ) ); echo '</xmp>'; // DEBUG
 		// echo '<xmp>'; print_r( get_query_var( 'page_slug' ) ); echo '</xmp>'; // DEBUG
 		// echo '<xmp>'; print_r( get_query_var( 'post_type' ) ); echo '</xmp>'; // DEBUG

 		return $tpl;
 	} // End process_rewrite_request()
} // End Class