Well, technically we have never left with extra cash, but if my wife applies the coupons for some items to their maximum potential, the cost of those items on the bill is a negative number, which is subtracted from the remainder of the bill.
As far as more than one coupon per item, it depends on the type of coupon. For example, Albertsons (which is a store that is working VERY hard to build a reputation of being coupon friendly) accepts a manufacturers coupon, but then they may have doublers or triplers in their own weekly ad, which can be combined. And they also have promotions like the current "Chill" deal, where if you buy 10 of selected frozen foods, you get a coupon good toward your next overall purchase (in increments of $5, $7, or $10, I think). So, that makes it possible to apply discounts from three separate coupons to the price of an item.
In fact, she made a second trip to the store today, because she had a Chill coupon, plus a manufacturer coupon for some more frozen fruit juice concentrate. She bought 10, and because of the coupons she used, she only paid $.09 per can of juice. But, on top of that, by buying ten, she got ANOTHER Chill coupon worth $10 toward her next purchase. So, in a sense, they paid us $.91 per can of juice today.