Some older motorhomes accomplished the dual brake/hydrovac setup by using two small-displacement hydrovacs, one for each axle. It's an overly complicated system with minimal advantage over a single system. These old rigs ran millions of miles with maximum loads (usually overloads) and didn't create much mayhem on the highways from brake failures. Keep the original system in good shape and keep the shoes adjusted properly. You'll have far more braking capability than you'll ever need, unless you intend to run the same kinds of loads on a daily basis that the truck was designed to haul. Just watch out for the problems they had when running unloaded back then- - - - -rear wheel lockup from having too much brake on the rear axle.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!