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.

This would be a cool thing for those with multiple cards to try as an experiment. Requried careful balance checking, of course, but most of us are anal enough to do that anyway (although I really only seriously check EQMs since I have over 1MM spendable miles in the kitty).

Cheers.