just an ergonomic theory on the brakes...
wasn't the wheelstand, part of the appeal of this vehicle?
If two levers brake the rear wheels, while leaving the jackshaft to spin (providing rotation power to the rear axle drive...the rear wheels lock and the drive gear rotates through the confines of the chain, forcing the machine to rise in the front. It's like a guaranteed traction, to perform the wheelstand, instead of just spinning the tires...Plus with the weight of the driver, that far ahead of the fulcrum (rear axle), that's a lot of power and force needed without some help...
the jackshaft brake, it seems would be the main brake.
I can see the braking individual wheels to aid steering as well, and maybe for power turns, with experience.
just a theory...