how to add multiple menu in custom menu widget?

You can find the WP_Nav_Menu_Widget class in wp-includes/default-widgets.php:1059

1. Simply copy the class (with new name – Example: Foo_Nav_Widget) to your functions.php file.
2. Edit the form method and replicate the select option (with new id/name) and any variables associated.
3. Edit the widget method, replicate any associated variables and call your second nav menu underneath the current nav menu.
4. Register the new widget using register_widget(‘Foo_Nav_Widget’);