Your registered substitute will also be able to set up their own passkey so that they are able to log on securely as well. Where you have a substitute who is not yet registered, they will still be able to log in to your account via the original method by requesting a One Time Passcode.
If they try to use a passkey, they will be required to reset the Passkey each time they access the app. You will need to give your substitute the One Time Passcode every time they log onto the app.