> I do love the cloud version of passkeys, but I also have a backup YubiKey

How does this work, exactly? Does you cloud password/key manager allow syncing to the YubiKey?

Or do you register a second passkey that you store on the YubiKey whenever you create a passkey?

If it is the latter, do all services that allow passkey authentication also allow registering multiple passkeys? How many?

> could do two backup YubiKeys [...] keep one YubiKey in a safe deposit box

If you register a new passkey on the primary, do you then have to take the backup YubiKey out of the safe deposit box to put it on it as well?

I haven't seen this kind of question answered when sites prompt me to use passkeys instead of passwords.