Why is it possible to link backup cards only once?

For security reasons, backup creation and private key cloning can only be done once. You should therefore be sure of the number of copies of your private key you want to create.
As the cards operate without using the company's servers, the cards know nothing about each other. It is only when creating a backup that the private key is copied to the number of cards you have chosen (2 or 3).
If backup cards could be linked on multiple occasions, an attacker could make copies of your card without your knowledge.