You should always add at least two keys for every service in case you lose the first anyway. That was the case even before Apple passkey so just keep doing the same.
This isn't a great solution, as in order to enrol in new services you have to either retrieve your backup key from its safe storage location, or keep the backup key with you at all times (which defeats its purpose).
And if you do move away from Apple's Passkey to your second key, you'll want to buy and set up a new backup key. So have to do the tedious mass-enrollment anyway.
