Big problem with this is that enrolling the secondary passkey requires the authenticator to be present. This is super inconvenient and risky as it always requires both authenticators to be present at the same machine/physical location, exposing both to local, physical threats (faulty USB ports on your machine frying anything you plug in? Congrats, you've now fried your main and any backup authenticators before you realized what was happening).

Ideally, you should be able to get an authenticator's public key and be able to enroll one without presenting the authenticator itself, allowing you to keep it in a safe/etc.

This would enable an easy workflow - enroll main authenticator as normal, then enroll your safely-stored backup by pasting its public key. If you lose your main, go to your safe, get your backup and "promote" it to primary and enroll a new backup one which goes in the safe.