This is one of the main design problems with 2WD bicycles and motorcycles. You might consider wiring a suitable resistor into the rear wheel throttle signal wire. I'd consider starting with a 5-10% decrease in rear wheel throttle signal voltage (you might try testing this with a variable resistor). An overall decrease in rear wheel speed would seem to limit the apparent advantages of having a 2WD bicycle. However, the front wheel would (hopefully) still offer traction if, and when, the rear wheel loses grip. Btw, 2WDbike designs offer little, if any, increases in top speed. Improved acceleration and traction are what 2 and 4WD designs excel at.