Hello Mateusz, so by default these are not bound to each other, there are only a few ways this could happen. If they are both related to a userid in the system you could actually join them on ContactOwnerUserID and CustomerUserID... however if they do not share a common user, they are in no way connected. There is built in functionality to create a new user on checkout, if you took advantage of this then you should be able to connect the contact and the customer through this common ID.