The advantage of the Dual Master cylinder is with a 3/4" bore the output pressure is 1400 psi and there is no wasted energy when using a balance bar. This is a copy of a link from the Brake Article on the internet and I quote (Dual MC's):

........."There are major advantages to using dual master cylinders: (1) Smaller diameter master cylinders can be used to increase output pressure. The design allows the application of two master cylinders being applied at the same, thereby doubling the volume output. Because of this high pressure output you will not need a vacuum booster. If you are running any type of camshaft, chances are you do not have enough vacuum to run the booster anyway. (2) The balance bar eliminates the use of a proportional valve and gives you the optional remote adjustment. (3) The remote fill applications deletes the need for residual valve normally used when the reservoirs are lower than the calipers.

When calculating the output pressure of each master cylinder you can not say that applied pressure is “shared” equally between the two (2) master cylinders. If the two master cylinders did not have a balance bar between them and the application of force was always equally distributed this statement would be true. The balance bar allows the applied pressure to be distributed unequally.

Example:

6:1 ratio pedal assembly
¾" master cylinders
Applied force of 100 pounds with your foot

The formula shows that this combination produces 1359 psi, however if you apply the 100 pounds of force to both of them equally it will only produce 50 percent or 679.5 psi.

What the balance bar allows you to do is apply 65% of the force to the front and 35% to the rear so the actual output pressures would be 883 & 475 psi.

This is how you are able to obtain maximum braking with the same amount of applied force. When you are using a tandem master cylinder (OEM type inline bore) the output pressure is equal in both ports and the only way to reduce the pressure to the rear braking system is through metering (distribution block, combination valve or engineering in the master cylinder) or proportional valve. This works fine when you have more than enough pressure with a power booster but when you are using a manual master cylinder this energy is “wasted”.

The easiest way to test this "wasted energy" is to apply 100 uniform pounds of pressure to a 6:1 pedal ratio and measure the pressure at the front calipers and the rear calipers with a pressure gauge. You will find that you will not have 763 psi you will have a reduced amount directly related to your proportioning or reduced pressure in the rear. If you reduce the pressure in the rear by 15% the out pressure in the front system will only have 648 psi at the gauge. The 648 psi is not taking into account "Friction Lost". Friction Lost is the amount of pressure lost from length of travel and the size of the piping.

The only way to know the effective actual output pressure of your system with a proportional valve or distribution block installed is to measure it with a pressure gauge when using a Tandem MC setup. You get 100% when you do not have this restriction on dual master cylinders. So how many pounds are you actually pushing with your foot when you use a Tandem MC?

The only disadvantage is higher cost..... but what is your car worth? What is your life worth?

Everybody has their own preferences, priorities, driving styles and disposable income (for our babies i.e. cars). Just give a formula or Physics reason why a Tandem MC works better than a Dual MC setup. FYI you can look in "Handbook of Physics" by Condon & Odishaw for any Physics rebuttal you might have. To an Engineer give him Physics and Math or give him a black and white answer, if you can't prove it on paper he won't believe you.

This setup is not for everybody, it is only another solution to no or little vacuum on the muscle cars we are building today. We did not have the horse power or speed 10, 20 or 30 years ago, so we did not need as much braking as we do now. With today's technology the sky is the limit and the only thing holding us from going faster is the tires, suspension and brakes not so much the engines. Band-aids like hydro-booster and vacuum canister/pumps are just one more thing to go wrong.