is there a possibility to mirrow 3A Mini with 3A just to have a backup key?
Short answer: no.
Long answer: Not a mirror, but some kind of backup usage is possible:
- For FIDO2/U2F, you cannot have the same keys on both Nitrokeys. You can either depend on websites allowing you to register more than one security key, which sadly is not very common. Or you can usually add a one-time password as an alternative second factor. Then you can store that one-time password on the other Nitrokey.
- For GPG keys, you can create your key in software and then move the private key to both of your Nitrokeys. If you created a key directly on one of your Nitrokeys, your out of luck.
- For static or one-time passwords, you can just add them to both keys.