different menu for mobile theme

i’m working on a mobile version of a site and have 2 themes – a desktop version and a mobile version. i’m using the Device Theme Switcher plugin to load the mobile theme when the site is viewed on a mobile device. i’m also using the Theme Test Drive plugin to proof the mobile theme before it goes live.

i created a second menu that i want to show only on the mobile theme, the first only showing on the desktop version.

each theme has different code in functions.php and header.php to load the menu i want for each theme.

everything is working great, but when i switch themes in the backend, the dropdown under Menus/Theme locations changes to blank and i have to keep re-selecting it every time. any idea what might be causing this?

i have this code in my DESKTOP theme functions.php

register_nav_menus(
	array(
	'menu'=>__('Menu'),
	)
);

and DESKTOP theme header.php

<?php
            //define main navigation
            wp_nav_menu( array(
                'theme_location' => 'menu',
                'sort_column' => 'menu_order',
                'menu_class' => 'sf-menu',
            )); ?>

i have this code in my MOBILE theme functions.php

register_nav_menus(
	array(
	'mobile-menu'=>__('Mobile Menu'),
	)
);

and MOBILE theme header.php

<?php
            //define main navigation
            wp_nav_menu( array(
                'theme_location' => 'mobile-menu',
                'sort_column' => 'menu_order',
                'menu_class' => 'sf-menu',
            )); ?>

is there a better way to set a different menu for a mobile theme?

thanks!