I use to change a lot of drum brakes and as I remember the primary shoe (one towards the front) had less surface area and did not do as much stopping as the secondary which was bigger in surface area. The primary shoe contacts first and forces the secondary shoe against the drum. Can't give you all the design reasons of why they are designed this way.

The shoe facing the front of the car is always has the *shorter friction surface. The shoe with the longer friction surface always faced the rear of the car.

brake shoes help slow or stop the moving outer drum