Ok, thanks to SutterCane
, I was just able to get both to work online, but you can't stack both coupons on one item. You need to have three items in your cart, all of which total $100 or more. The most expensive item will get the $25 off, the second most expensive will get 20% off, and the third will have no discount.
Here's what I did=
1. Added 7879 Hoth Echo Base ($94.99)
2. Entered the coupon code for 20% Off
3. Added 7641 City Corner ($59.99)- this removed the 20% off, made 7899 $25 off
4. Added a Series 3 Minifig ($2.99)- this made 7879 $25 off, 7641 was now 20% off
Cart total (with free shipping since it's over $75)- $120.97 plus tax, a savings of $37 off TRU prices. Of course, your savings will increase if you buy more expensive sets that do not have TRU markup. Replace 7641 with 7692 Podracers, and your total will be $148.97, $44 in savings.
The third item in your cart can be one of the small polybags like the Harry Potter minifig, nothing fancy. So the coupons will not stack, but you can play with your cart to use both in a single order. Not as good as a 7965 Falcon for $50-60 off, but you can get both deals to work together if you order multiple sets.