I've recently bought two Yubikeys Neo which I'd like to use primarily for encryption and authentification by using the smartcard feature with GnuPG. I've read a few how-to on the subject (most notably here and here) and I've managed to generate and export to the Yubikey a keyring with the 3 subkeys for signing, encrypting and authenticating. Using these, I was able en encrypt a file using my public key that I could only decrypt by pluging in the Yubikey. Great.
Now I want to duplicate this setup on the other Yubikey to act as a backup in case I lose my main device. Since gpg's "keytocard" command deletes the local keys I was only able to export the same keys to the other Yubikey by making and restoring a backup of my .gnupg directory (which doesn't seem right), and even so, when I try to decrypt my test file, gpg asks specifically for the key with some serial number and it won't decrypt with the backup key.
Is there a way to use two different Yubikeys as a backup like this? If not, what is the best way to proceed to achieve this? (meaning : keeping a backup key that I can use to decrypt my files in case I lose the main key).