Well designed platforms and auth frameworks allow adding multiple passkeys. The problem is this requirement was never coded into the passkey standard.

That still requires recreating every passkey, even if both can exist at once. If I want 50 sites to use a passkey on macOS and Linux, I need to go through the setup process 100 times, and all 50 sites will likely handle it slightly differently.

It also requires foresight if switching. I ran into this with 2FA as well. When I got a new phone, after setup it offered to wipe the old one, I declined. When I went to open my 2FA app for work on the new phone, it needed to be setup. I needed the 2FA on my old phone to login to the portal and setup the new phone. Had I wiped my old phone when prompted, I would have been locked out and on the phone with the help desk. This same scenario can happen with Passkeys.