Originally Posted by brp
I see...buy on one card one month, return on that card the next month while buying on the other. This way one card each month will have the negative balance
You might be able to do a return that way sometimes, but most stores insist that the return be made on the original card. Also, the negative mileage balance on your mastercard doesn't just go away. If you're closing the card or retiring it then fine, but otherwise your new mastercard purchases will be applied toward the negative. There's no net gain here.