redirect to other "view" of same post

Suppose a single post (single.php) has an URL example.com/post-x/
On this page there is a link which is example.com/post-x/print/
In other words, the same URL but with an added /print/ slug.
When I click on that link I want to show the same post, but with a different layout (coupon).

I suppose I need to use template_redirect so what I have so far is this:

add_action("template_redirect",  "couponRedirect");

function couponRedirect() {
	global $wp_query;
	if ('print' == $wp_query->query_vars['name']) {
      if ($wp_query->is_404) {
         $wp_query->is_404 = false;
      }
      header("HTTP/1.1 200 OK");
      include (TEMPLATEPATH . '/coupon.php');
      exit;
   }
}

But this does not display anything (empty page). Now I am guessing I need to include a query? Can someone point me in the right direction as to how to do this?