IMHO two brakes will help on a long, fast downhill, but not on a level maximum stop.

Interestingly, to me, I have formerly ridden with a Cane Creek Super Light single-pivot brake in front, and it did not have the power to really stop the bike, but, it would also not put me over the handlebars.

That particular inadequate front brake started me learning how to back pedal; and, as a result, I learned more about back pedalling than I had previously imagined existed.