Hi Markus
Unfortunately, it is not possible to choose multiple categories at the moment. I will work on this improvement next week. However, it might take me 2 weeks to get it completed.
Maybe for now, you can go to Events Booking -> Configuration, find the config option "Assign Custom Fields to category instead of to events", set it to No. Then instead choosing a category for custom field, you can choose multiple events (any events you want).
That is the only solution for now. If you want to select multiple categories for custom field, you will need to wait for two more weeks
Regards,
Tuan