Hey Martin,
You do not need both plugins. They are sold separately and work wholly separate of each other. All you need is to have the core/free version of the plugin active, and then any add-ons you’d like (or none! :)).
Next, the Community Events add-on does not create a second calendar or anything – it uses the same calendar that the free/core version creates, but simply adds a page where users can submit their own events to it.
Definitely check out the link I shared earlier: https://theeventscalendar.com/product/wordpress-community-events/
Watch the intro video, check out the “New User Primer”, and generally just explore that product page to learn more about what the add-on does and if it’s useful for you.
Let us know if this helps!
Thanks,
George