How can I produce multiple webpages with a different output based on one entry/Page/custom Page?

Query string parameters are a helpful friend in this situation. It will allow you to keep the same single-$posttype.php file for both formats of the page.

So if someone visits your page using the URL:

They will see your normal webpage for visitors. But if someone uses the URL:

They will see something completely different.

This is accomplished by using the $_GET global variable provided by PHP. Documentation is here.

So you could do the following using a single-event.php page (providing event is a post type):

$is_seminar = $_GET['seminar'];

if ( $is_seminar === 'true' ) : ?>
  <h1>Custom look for seminars.</h1>
<?php else : ?>
  <h1>Normal webpage</h1>
<?php endif;

Or if you want a little bit of a cleaner look to your URL, take a look at Get URL query string parameters using $_SERVER['QUERY_STRING']. Also see How can I make different page templates for one category? for another alternative solution using your post’s meta info.