Coupon stacking is not to be confused with doubling coupons. Doubling coupons means the store will take the value of your coupon and double it. Coupon stacking means using two coupons on one item.
Each store has its on policy on whether they allow stacking. If they do permit stacking, this will allow one manufacturer coupon and one store coupon per item. Remember, you have to determine if your coupon is a store or manufacturer coupon before you try to stack.
This is a store coupon (or in-ad coupon)
You can see the store name at the top, meaning it is good only at Walgreens. This can be stacked with a manufacturer coupon.
This is a manufacturer coupon.
You can see the words Manufacturer's Coupon at the top. This coupon is provided by the makers of the product. This can be used at any store and stacks with a store coupon.
This is also a manufacturer coupon.
Just because a coupon has a store logo on it, it does not automatically make it a store coupon. You can see the difference in these two Target printable coupons. One is for Target use only, one is for use in any store, but provided by Target.
This is a Target Only web coupon.
I hope this helps you navigate the world of coupon shopping!