The simplest way to describe a split braking system is to visualise that the pressure to the brakes are applied to the wheels on separate lines carrying a special hydraulic fluid. This is done for several reasons, safety among them. There are two basic types of split braking systems. One is a front and rear split system. The second is called a diagonal split braking system. There are different reasons for both, though they share the common goal of straight-line braking.
Since braking systems work based on hydraulic fluid that runs through lines that apply pressure to braking cylinders in the wheels, a brake line failure may prove catastrophic. If the line leaks or breaks for any reason, pressure is lost. Without pressure when the brake pedal is applied, the braking cylinders in the wheels couldn't apply the force needed to stop the wheels from spinning and, in turn, not stop the vehicle. This applies to both systems.
Sometimes, split braking systems use two master cylinders to control each conduit (brake line). This is akin to a double safety backup. Since master cylinders exponentially increase the force of the hydraulic fluid to the brakes, should one fail, the vehicle still has stopping capability, albeit the stopping distance is greater and less even (more difficult to control). On the other hand, with two master cylinders, the system has one more additional part that could fail, but engineers consider that the safety features outweigh the potential for failure if properly maintained.
The diagonal split concept comes from the fact that the left rear and right front brakes are on one hydraulic line while the right front and left rear brakes are on another. The diagonal split system, because it maintains braking ability for both a front and rear tire, is easier for the driver to control the vehicle in emergency brake failure. In a nondiagonal system, all the braking power would transfer to either just the front or back tires, increasing the likelihood of skidding and possibly losing control.
Front brakes on vehicles typically sustain about 70 per cent of the braking load, which makes sense since vehicles are generally travelling forward at high rates of speed. This varies, at times substantially, if the vehicle is hauling heavy loads. A front and rear split braking system is less desirable when the vehicle is expected to be hauling loads because if the front system fails, the remaining rear system may be inadequate to safely stop the vehicle. It's one of several reasons why diagonal braking systems are generally preferred in most modern automobiles and light trucks, particularly those expected to be hauling horse trailers and the like.
Science is Good
Whichever split system is installed on your vehicle, you can find some comfort with the advent of anti-lock braking systems (ABS), which were a significant development to preventing unsafe stopping conditions by keeping the tires in relative synchronisation. By keeping the revolutions of all the tires electronically within a 5 to 10 per cent rolling velocity, it helps the driver keep the vehicle under control while it is brought to a stop, minimising skidding and the loss of control that could easily lead to accidents. Even without brake failure, it helps safer stopping by more equally distributing the braking power of the vehicle.