We belong to both UA and AA VISA dining programs. From the same restaurant using the same VISA, we got UA miles one trip, AA credit another trip. Any idea how these systems choose?

Why did you use the same visa card for both programs?

Per the T&C's of the program, a card can only be registered with one program,so somehow it is deciding for you what program to dump the credit into. (you did know you are not going to get credit to both programs for the same dining event?)

I believe that the OP will find that when he registered the card in the second program it was removed from the first program. Indeed this is what happened to me. I was registered in the UA program but decided to accumulate miles with AA raather than UA so I registered the same card with the AAdvantage program. That card was automatically removed by the system from the United Mileage Plus program. If you only a re one credit card they make it so that you can only be in one program at a time. Otherwise you could double dip.

i think discover cards are for free. i use a discover card for the second airline to keep the miles alive from year to year.

Do you have the same Discover I have the one with rotating categorys 5% back ? if so which design do you have? do you use Discover for business?
or just on vacation travel?

or do you have the miles Discover card or open road Discover ? how do you like your Discover?

I have the original Discover , mine is the black design. I don't travel on business. I like my Discover. no miles or open road for me.

If this really happen, you must have registered the same card at both programs, and timing was such that the first dine had already come in to the first program, then you registered at the second program, and then the second dine came in.

The way it works is that the program to which the card is associated at the time the dine comes in (which I think is typically a day before you see it online) is the one that's used.

I've certainly done this on purpose, when I had a promotion on a particular credit card, but which airline I wanted to earn changed from one month to the next as one lost its promo and the other gained a promo. With a couple day gap (during which it's sort of hard to predict which program a particular dine will credit to), I was able to move the card from the first program to the second program.

But the programs freak out if you remove all cards from a program. So it's best to always keep at least one card in each program. But if you wish, it can be cards that you have in a drawer but never use (since RN will never charge anything to the card). You also don't need to delete a card from an RN account just because the card is expired or cancelled; knowing this can help you always keep a card or two in each account, while also moving your favorite card between account to "follow the bonuses".