The idea is that if the front or the rear line, or lines rupture or burst, the other will survive and give you enough braking power to stop the vehicle. Needless to say the chance of both going out at once is unlikely. With a single master cylinder, all you have left is the emergency brake.
To answer your question specifically, a dual master cylinder is twice as safe as a single master cylinder.
It is a good thing to get them to run well and attain highway speeds, but it is a better thing to be able to stop when needed.
