Holy smokes... I never even thought to check that. I had assumed that only a token or decryption key used by the app was on the Yubikey, I didn't realize that the whole deal was on there.

That leads to a very intersting question: what if I want to use two Yubikey Neo's as a backup if I lose one of the two keys?
It would be cool if the Android Authenticator-App could sync the config to more than one Yubikeo Neo.
Any plans?

If not that means one can use only one key at a time. Moving from one key to the other means deleting every single totp-login stored and re-create it with the new key, doesn't it?

Speaking of the 6 or 8-digit OTP codes only, used by systems like Google Authenticator or Yubico Authenticator (this does not apply to the yubico-standard-OTP that used a long modhex string)...

So, for HOTP 6- to 8-digit OTPs you cannot, since the shared moving value is a stored counter that cannot be kept in sync between two tokens.

Luckily most systems are implementing TOTP 6- to 8-digit OTPs these days. You can have multiple tokens in that scenario, since the shared moving value is the current time (at authenticating server and at phone/PC which will be in sync regardless).

The trick is that if you want to use multiple tokens with a TOTP credential, you have to capture the secret key before it's pushed into the first token and then erased from your screen. I usually do this by telling the system I want the text value (instead of the 2-D barcode) and then write down that value. Then use the "Add Account Manually" option in Yubico Authenticator twice, once for each key. Then destroy the written-down value.

Brendan