It's really very simple if you just look at the Federal Regulations for automotive brake systems published in 1967. Although almost all automotive manufacturers had adopted the dual master cylinder before that date, the actual Federal requirement wasn't adopted until that year. The following is the section that requires a dual master cylinder system and what the system is required to do.

49 CFR 571.105 - Standard No. 105; Hydraulic and electric brake systems.

S5.4.1 Master cylinder reservoirs. A master cylinder shall have a reservoir compartment for each service brake subsystem serviced by the master cylinder. (where brake subsystem means front or rear brakes) Loss of fluid from one compartment shall not result in a complete loss of brake fluid from another compartment.

The entire reason for the requirement for dual master cylinders was to provide emergency braking if pressure is lost in either subsystem and that's exactly what dual master cylinders do. Single master cylinders do not have that capability. In dual master cylinders if you have a leak in the front or rear brake lines the other will still be capable of stopping the vehicle. The requirement for this is stated here:

S5.1.2.1 In vehicles manufactured with a split service brake system, (front and rear systems) in the event of a rupture or leakage type of failure in a single subsystem, other than a structural failure of a housing that is common to two or more subsystems, (master cylinder) the remaining portion(s) of the service brake system shall continue to operate and shall be capable of stopping a vehicle from 60 mph within the corresponding distance specified in column IV of table II.

Because the loss of rear brakes may not be immediately noticed since most braking is done by the front brakes, a requirement for a pressure switch was adopted but this happened in subsequent years, not in 1967.