Which card are you using now? In general, banks that issue personal MasterCard and VISA use the identical number for every card on the account. This makes it cumbersome to match charges with individuals and it means that if one card is lost or stolen every card needs to be replaced. The solution is to use a business MasterCard or VISA, because every individual card will have a unique number.

American Express cards issued by the company assign a unique number to each card on personal accounts, but Amex-network cards issued by banks may use either system.

Delta is closely tied to American Express. United is even more closely tied to Chase. There is no card that earns miles in both programs at a competitive rate. The closest you can come is the Starwood Preferred Guest credit card issued by American Express. Starpoints are transferable to three dozen airlines, most at the rate $20,000 = 25,000 miles. United is transferable at half rate ($25,000 = 12,500 miles), however Starpoints can be transferred at full rate to US Airways, and their miles can be redeemed for travel on all Star Alliance airlines, including United.