Insert menu into theme location depending on user logged in/out status

Okay, figured it out! I’ve amended my theme function as follows:

add_filter( 'wp_nav_menu_items', 'add_loginout_link', 10, 2 );
function add_loginout_link( $items, $args ) {
    if (is_user_logged_in() && $args->theme_location == 'top_navigation') {
        $items .= wp_nav_menu( array('menu' => 'menu-logged-in', 'container' => '', 'echo' => false, 'items_wrap' => '%3$s') );
    }
    elseif (!is_user_logged_in() && $args->theme_location == 'top_navigation') {
        $items .= wp_nav_menu( array('menu' => 'menu-logged-out', 'container' => '', 'echo' => false, 'items_wrap' => '%3$s') );
    }
    return $items;
}

So now, just gives me the <li> items I need and rendered in the correct position. Hope this helps someone.