I'm with Otto, absolutely no need for a dual master cylinder. I have probably put millions of miles on my rides in 75 years of driving with zero brake failure. Just check out the system if anything changes which is usually pedal height. My 37 Buick has a dual mc that is off a disc/drum car after I had full pedal-no brakes after driving with the drums under water. I drive my 37 border to border and coast to coast so likely run into situations very few experience in their old rides. I'm cheap so a dual mc is less expensive and complicated than proportioning valve and/or 2#-10# residual pressure valves.

Evan