How does the secondary credit card work?

If you choose to add a secondary card to your account, it will serve as a back-up card if the charge presented to the primary card is declined or not accepted in the zone in which you are trying to park. Serving as a back-up card, and not being the main card on the account, the secondary card will not prevent your account from becoming disabled if the primary card on file expires. If the primary card on file expires, but the secondary card on file is still valid, the account will still become suspended for expired card. A valid, non-expired card must become the primary card in order for the account to be enabled.