In fact, it is easy to understand. If you sign a contract and both parties agree that the signature or seal is valid, will the law say that your contract is invalid because you do not meet both elements at the same time? Of course not, so signing or sealing is enough. The law stipulates it just because of custom and to give the parties as much freedom of choice as possible. It is not to restrict you and ask you to have both. You can absolutely do both. Choose one!