[6volt]

While I can't say whether the 2 shoes were different starting thicknesses, I did record a single number for new thickness when installed. I believe it was 11/64's.

One way to visualize how the shoes work is think about how the shoes are kept from rotating with the rotating drum when the brake is applied.

In short, the brake assembly is kept from rotating by the rear shoe reacting against the big pin on the top of the backing plate.

But any friction force generated by the front shoe appears as additional actuation load on the rear shoe.

If each shoe was reacted on its own pin with its own wheel cylinder, there would be no PRI and SEC shoe. (There are brake systems like this!)
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I looked at my rear brakes today and they are installed properly (small in front) and they are wearing evenly with just a little more wear on the fronts.

I am suspicious of the effect of wheel cylinder condition on all of this. Ironically, on the rear brakes, both wheel cylinders were rebuilt in 1995 and while no brake fluid was dripping from them, when I pulled the boots back, a few CC's of fluid poured out.

So rear brakes with rebuilt wheel cylinders that were starting to leak, did a better job that new everything in the front done at the same time.

I really can't explain any of this.
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I remember back in the 70's some brake shoes came in a pink and grey lining and small and large. It was obvious they were using different material for the PRI and SEC.