A Hydrovac as I understand it is a remote mounted brake booster. It hangs on the frame wherever you want to put it, this system uses an ordinary master cylinder in the stock location.
If you had a dual Hydrovac system it would be two identical Hydrovacs usually mounted side by side on the frame fed by an ordinary dual circuit master cylinder.
To upgrade a truck that originally had either no Hydrovac or only a single one to now have dual Hydrovacs will involve finding a dual reservoir master cylinder with the proper bore and making or finding an aftermarket bracket to mount it. You may luck into a bolt on replacement dual circuit master in the later years, I'm not sure.
I think if you are going to update the MC and booster and no longer be original then a better solution than using dual Hydrovacs is to use a more modern master and booster bolted together as a unit. This will be a more reliable and cheaper system.
Even better yet though. If you already have a power steering pump then I highly recommend a hydrobooster, it is small, compact, powerful, and works off the PS pressure. I have had a few Hydroboosters, and really like them, doubt I ever use a vacuum booster again. (But they are not recommended for drum only brakes on a small vehicle, larger trucks are OK with drums though.)
Grigg
