GPG encryption subkey on multiple smart cards issue

I guess you will have bad luck, and this is not supported by GnuPG. When using OpenPGP smart cards, a secret key dummy is stored in your keyring, holding a reference to the smart card it is stored on. The secret key subpacket looks like this when displayed through gpg --list-packets:

:secret sub key packet:
        version 4, algo 1, created 1358985314, expires 0
        pkey[0]: [2048 bits]
        pkey[1]: [17 bits]
        gnu-divert-to-card S2K, algo: 0, simple checksum, hash: 0
        serial-number:  01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef
        keyid: 9FF7E53ACB4BD3EE

Peeking at the code, it does not seem to me more than one serial number would be supported.