Brake master cylinders provide little (well, its the volume of fluid too) pressure to the brake lines. The master cylinder is simply a fluid reservoir and control for the hydrovac control valve. So no, nothing you bolt on the output of the master cylinder will work.. there simply is no system pressure there.
All system pressure is generated by the hydraulic cylinder at the hydrovac. The pointy end.. that the brake line plug's in to.
My manuals show the largest Series 3 vac only having a 3.5cu/in hydraulic cylinder displacement.. that's total displacement... a well adjusted brake system will not use nearly all of that volume at between 800-1200 psi depending on model.
If you have your vac PN I might be able to get the displacement and line pressure out of my manuals (as long as its a Series I, II or III made by Bendix) and with that info you could do as Lincoln suggested... install two vacs each dedicated to the front or rear. The master cylinder plumbing would be simple, and the hardest part would be finding a spot on the frame rail for a second vac and the plumbing up to the intake.
So you could control two vacs with a single master cylinder, but for a true, dual circuit setup you'd want a dual port master cylinder to truly isolate each hydraulic loop.
