I just tried, and after adding the 2nd DVD set, it listed a line item for the discount, and it deducted $10 from the first DVD set that was in the basket.
I think I know what was going on. I was looking into the yub.com thing which has this extra 1% off at buy.com if you order through that site. I think the promotion from that must have cancelled this promotion. Then when I tried to go in separately, my account was still confused. When I just started out fresh it appears to be working.
Patrick, thanks for telling me how it should appear.