If you slow the bike from either the front wheel or the back wheel, energy is removed from the whole bike. There's little to no benefit from harvesting from both wheels.
All the resistance is from only the rear wheel. So its like rear wheel breaking, and potential lock ups in the wet. Surely its better to spread the load. Also means twice the surface contact. Wouldn't that be better, perhaps safer, to ride? Especially in the wet.
But the extra cost and complexity, even weirder look, extra wires.
For emergency braking, keep the regular front brake. For minor speed adjustments the rear brake will get the job done.
