Unfortunately, the Stocard Backup feature is limited and not made for synchronization yet. After you restored the backup on your second device, there will be no automatic synchronization between your devices (although they use the same backup account). That's why, when you add a card on one of your devices it won't be automatically transferred to your second phone.
However, what you can do is to refresh the backup on your main device on which you added the new card. Afterwards, simply sign in with the same details on your second device. After logging in your cards (including the new ones) are automatically restored or a pop-up message appears with the options to either "Restore old Backup" or "Create new Backup". Please select "Restore old Backup" and all your cards should be available on your second device.