Does Solo Keys enable that?
Generate whatever you need on your PC and then load it into as many keys you feel like.
But even if you don't want to use the popular passkey implementations, you can still easily register multiple authenticators which mitigates the risk of losing one. On a new site, I'll register my hardware FIDO key, my phone as a passkey, and my laptop authenticator (either touch-id on MacOS or tpm-fido on linux machines).
Soon there will be other passkey implementations that will also support syncing and backups (1password for example is working on this).
Don't settle for a phishable authentication method.
We'll see how it plays out but I'm not optimistic this is going to end well.
Why? Its all open standards. There are already a number of independent implementations in the works.