In the end I approached the issue in a different way. I limited the depth of the dropdown menus to the 2nd level, so just 1 dropdown appears now.
I edited navigation-top.php adding the “depht” argument to wp_nav_menu:
wp_nav_menu(array('depth' => 2));
Anyway, I think that the behaviour of the top menu should be analyzed in the next versions of the theme.
