(FORTUNE Magazine) – Joint accounts are the bane of many a household, but not in the credit card business. Co-branded credit cards -- those issued jointly by businesses and card companies, like those shown here -- are gobbling market share. These cards typically offer discounts on cars and merchandise, or free airline tickets, in proportion to the amount you charge. Winner in the pairs competition: General Motors' GM Card, issued through Household International, has grown to nearly 8.8 million pieces of plastic in just 17 months, according to Credit Card News. GM's card lets consumers roll up rebates for car purchases with each dollar charged. So far, auto-buying cardholders have cashed in 123,000 rebates, worth $40 million. Other recent partnerships include Shell Oil and Chemical Bank, and Apple Computer and Citicorp. The newest player is Japan's JCB International, which has a partnership with Household. JCB is likely to try to sign up business travelers and offer them theater tickets and other services, just as it does at home. With co-branding, MasterCard International is turning into a hard-charging house. Partnered cards now represent 80% of the No. 2's new business and 38% of its volume. Through the third quarter of 1993, MasterCard's volume increased 23.5%, to $97.35 billion; No. 1 Visa International rose 16.6%, to $158 billion. Co-branded cards are the shape of things to come, believes Spencer Nilson of the Nilson Report, an industry newsletter. Says he: ''At least half of all cards in the country will be co-branded. There's nothing going to stop it.'' The paired cards are prompting consumers, who now average three cards, to condense their spending on one card to get the goodies.

CHART: NOT AVAILABLE CREDIT: FORTUNE TABLE/SOURCE: NILSON REPORT CAPTION: TOP CO-BRANDED CREDIT CARDS Sources of gas or computer discounts