By "you just use multiple", you mean I would have to generate extra private keys, add them to authorized_keys, and store them somewhere other than my mac, right?
Because just by using multiple keys I would still be locked out if all of them were stored in this 'Secretive' app.
If you are making multiple copies of a private key in different places, why wouldn't you just keep a different private key in each of those places along with its corresponding public key though?
That way, you can remove trust from just one of them if you (e.g.) have your computer stolen.
Well, not exactly, I use multiple physical keys. Yubikeys and OpenPGP smartcards in my case. I use multiple different types too (the OpenPGP cards are quite cheap too).
Because just by using multiple keys I would still be locked out if all of them were stored in this 'Secretive' app.