Hi Benjamin,
It’s more of a WooCommerce question than a WooCommerce Tickets one.
Applying discounts and other promotions in situations like these is something where if you can’t find a straightforward way to set this up even for ordinary products you will probably need to put a customization in place.
Of course, lots of addons exist for WooCommerce so it is also possible there is something out there that will do this for you.
Not sure this is something where we are going to be able to offer a great deal of guidance, though (unless of course you have specific questions relating to how WooCommerce Tickets itself does things?).