You guys seriously haven't been able to figure this one out yet? 😀
When you're buying an iPhone online at the online Apple/AT&T store, you can make two purchases on one card as long as you don't check out until you've added another iPhone, which must use a separate AT&T (or any carrier) line in order to be eligible to be placed in your cart. It's the same as two people showing up in an Apple/AT&T store using one card (say, a father and son) to purchase 1 phone per person, one phone per line, except you're vouching for yourself using your number and relevant account information over the internet.