The setup mentioned in the guide you linked deos NOT offer drive decryption (which is done at boot), but merely two-factor authentication for user login once the drive has already been decrypted.
If you want to use the yubikey to actually decrypt the drive, you’ll need something different, for example:
- Using a YubiKey as authentication for an encrypted disk | End Point Dev
- password - Yubikey Two-factor Authentication Full-disk Encryption via LUKS - Ask Ubuntu
With this out of the way, the guide specifically answers your question:
If you have multiple YubiKeys for backup purposes (in case a yubikey gets lost, stolen or breaks) you can write the same settings into other YubiKeys. You can choose “Program multiple YubiKeys” in the program, make sure to select
Same secret for all keysin this case.