Enrolling with/registering several devices. Webauthn is pretty cool if not a bit complicated.

For example, passkey on iPhone and on the same account register a second key such as a yubikey.

That’s how my github is configd, for example: my phone and MacBook can auth w OS passkeys. If I need to login on another host, like my windows host or in your example at the library, the yubikey is another registered hardware authenticator.

Worth adopting/looking into imo