From what i know about this is that there is some obscure federal banking regulation that says refunds must occur in the same form as the item was paid for. i.e. check for check, cash for cash, credit card for credit card. Now there are some variations on this. If you paid by check and the check has cleared then you can get cash back, if you paid by cash they can give you a check back (however, you can demand cash), if you paid by credit card then you can only get a credit back.

The point of this regulation is to prevent a person from charging say $10,000 worth of goods, and returning them later for cash, then stiffing the credit card company by not paying the bill. The credit card company ends up out 10K, but if it credits back then they are not out the 10K. However in the situation you presented, and as far as UA is concerned, FOP is credit card therefore refund should be Credit card. They will credit the last card on the account the full 10K. The other card you will have to pay in full, if you don't then they can take what ever action necessary agianst you. If you tell them that the ticket was refunded, then they can call UA and find out where the money went, and try to collect from the other CC, but in the meantime you are on hook. So the best bet is to keep using the same card for both transactions to avoid any real problems.

So basically the last card in the record gets the full refund. Also I know that you cannot pay for a ticket using two different credit cards. Therfore they should not be able to do a two card refund.