Just did a purchase with 3 games. Here are the numbers. Maybe you can make sense how it stacked.
Gears of war 4: 14.99
Disc -4.50 promo
Disc -1.5 power up 10%
If you're getting $9.44 pre tax, then they are not stacking. You're getting one discount off the full and then the next one off the discounted price. It would be $8.99 if you were getting both off the full price.