These two are also at play:
http://en.wikipedia.org/wiki/Unsprung_mass
http://en.wikipedia.org/wiki/Gyroscope
In my (really irrelevant) example the bike was measurably faster on track with a single disk BUT it still had plenty enough braking power for the task at hand.
(It was then about 170kg with me on it - braking from around 90kph to about 40kph, the situation may well be different on any other given example.)
I've ridden bikes with not enough brakes, you can tell pretty quick, which (If easy enough) is why I'd plan to start with one and add another if required.
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