Combining two WordPress menus into one

menu.php

<?php

// two WordPress menus combined into one.

// first menu.
$menu = wp_nav_menu( array(
	'theme_location'=> 'secondary', // or whatever location
	'fallback_cb'	=> false,
	'container'		=> '',
	'items_wrap' => '%3$s',
	'echo' => false
) );
// include all of the menu items from the first inside the second menu.
wp_nav_menu( array(
	'theme_location' => 'primary', // or whatever location
	'items_wrap' => '<ul id="%1$s" class="%2$s">%3$s ' . $menu . '</ul>',
) );