I'd try to get the job done with one washer.
The mating surfaces on the wheel cylinder and the bolt have what's called a "phonographic finish" which is designed to bite into the copper and create a better seal than a plain dead flat and smooth surface will.
When using 2 washers you would have a less than ideal seal between the two washers.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-