80% of the stopping power is on the front wheels. Especially once you apply brakes and start slowing down, it shifts all of the weight to the front. One brake is more than enough to lock BOTH of the rear wheels. What more do you want than locked? If you dont believe me try this -- get going about 15-20MPH -- use only the foot brake to engage only the rears. It will take a long time to stop. Try again with the hand-lever and you will be quickly stopped.
The brakes are not the limiting factor, the traction is.
Heck, even try the above experiment on pavement. Front brakes are what stop these machines.