Even if you could stack discounts, they would not be added together to make 20% off. Merchants usually apply the first discount and then the second. So, for instance on a $100 purchase, the first 10% would be applied to $100 which would result in a balance of $90 due. Then the second would be applied to the $90, which gives a final price of $81. The end result is a 19% discount, not 20%.