Share the secondary card with another person. A supplementary card is a card attached to another credit card. These two cards will share a total overdraft, and you just need to share the supplementary card with another person. Credit card, also called debit card, is a credit certificate issued by a commercial bank or credit card company to eligible consumers. It is in the form of a card with the name of the issuer, expiration date, number and cardholder's name printed on the front, and a magnetic stripe and signature strip on the back.