Correct me if I'm wrong but I don't think you're meant to clone them per se. You're better off creating a unique passkey for each device, and if one is lost you can revoke that passkey without bricking your backup device as a login method at the same time.

That's how I treat my Yubikeys at least. The only downside with this is that some services (e.g. PayPal) only let you create one passkey.