Combines two WordPress menus into one.

combine-menus.php

<?php
/**
 * Combines the markup of two menu areas into one.
 */
 
// Get the markup list items in the first menu.
$menu = wp_nav_menu( array(
	'theme_location'=> 'menu-1',
	'fallback_cb'	=> false,
	'container'		=> '',
	'items_wrap' => '%3$s',
	'echo' => false
) );

// Display menu-2 with all the list items from menu-1 included.
wp_nav_menu( array(
	'theme_location' => 'menu-2',
	'items_wrap' => '<ul id="%1$s" class="%2$s">%3$s ' . $menu . '</ul>',
) );