As a WordPress developer with years of theme customization experience, I often enhance the Twenty Ten theme's navigation. It defaults to one header menu but supports multiples via register_nav_menus(). Here's my proven method to add a secondary menu quickly.
Open functions.php and locate this comment:
// This theme uses wp_nav_menu() in one location.
The following line registers the menu using an array:
register_nav_menus( array( 'primary' => __( 'Primary Navigation', 'twentyten' ) ) );
'primary': Unique menu key.
__( 'Primary Navigation', 'twentyten' ): Human-readable description.
To add a second menu, extend the array with a new key-value pair:
register_nav_menus( array( 'primary' => __( 'Primary Navigation', 'twentyten' ), 'secondary' => __( 'Secondary Navigation', 'twentyten' ), ));
This approach works for other themes and child themes too. Questions? Drop a comment below—happy to help from my expertise.