I tried enabling their "advanced security" programme on my account and it's currently refusing to continue without at least 2 keys configured.

The first "hardware key" is actually my Bitwarden faking a hardware key (I'm sure they'll start blocking BW because of this in the future) but it doesn't let me add a second one unfortunately.

It’s a security feature, Apple does the same when you register a security key. You must register two.

If you’re using real yubikeys, it’s protection against losing one. If you had two from the start, you’re not at risk of losing your only way into your account when one goes missing or is stolen.