How to order separated Custom Post Search results

What kind of control do you want exactly? There are, for example, plugins that provide a UI through which you can order posts by various parameters.

If you just want to order them all by one parameter that will not change, you can simply add it to your query (since you can order by multiple conditions):

if ($query->is_search) {
    return $wpdb->posts . '.post_type DESC, title DESC';
}

Edit: here’s another question you might find useful.