What would be the purpose of having multiple passkeys for the same account stored in the same BitWarden vault? You're going to have a backup key and store it in the exact same place as the primary key?
Multiple passkeys backed by different sources (password manager, iCloud, Yubikey, etc.) can serve as a backup in the case you lost access to your password manager, for example.
If a service provides the option for more than one passkey, I always configure several.
But that doesn't explain why you'd want multiple keys all in the same password manager. That seems to miss the point of the redundancy, like keeping an "offsite" backup onsite.