In GPG 2.0.26, when you generate the key on-card you can choose to make an off-
card backup of the encryption key. You are not given the option to make a backup
of the signing key.

Later, if you lose your smartcard and delete the secret key for it, then import
your backup, you can decrypt messages encrypted to you but you cannot sign
messages. Also, the backup is just the encryption key packet, without UIDs, so
most users can't import it if they're using GPG < 2.1, since these versions can't
gracefully merge keys.

This is not the behavior of gpg2 --export-secret-keys, which exports both the
secret signing and encryption keys AFAICT.