Try this css code:
1)in your theme's php page add following code
register_nav_menu( 'primary', 'Primary Menu');
register_nav_menu( 'secondary', 'Secondary Menu');
2)then call these two menus side-by-side in header.php
wp_nav_menu( array( 'theme_location' => 'primary' ) );
wp_nav_menu( array( 'theme_location' => 'secondary' ) );
Rest, you gonna have to do the css to make them appear in the same line and with different font size etc..
3)Create two menus and assign them to primary and secondary menu
I guess that should do it...Thnx!