Paul bunq plans are always for a single person. If you want to have a Joint account with 2 people, both of these people need to have bunq accounts with at least the Easy Bank plan. The Easy Bank plan allows to be part of up to 1 Joint account.
The Joint account in and of itself is not paid, but it's a feature that is only accessible with a paid plan indeed, and plans are always for a single entity (natural person or company).