Unless something has changed, it's not even possible to pay for a single ticket with 2 separate cards over the phone.

I ran into this about 3 yrs. ago, needed to put a certain amount on my work card and the balance on my personal card. Neither the EXP desk or Web Services could do it...both said it was only possible at the airport. Luckily that was back in the day when you could call any AC directly, so I called the RDU club and they were able to do it.

I'd do as you mentioned above, buy some AA gift cards with your other credit cards to pay the remaining balances over what you can buy with your Citi card.