[Wilburbeest]

Twin superchargers are sort of in the same category as twin charging (supercharger AND turbo). Its a cool idea and it has been experimented with a bit, but it doesn't really make sense for anything but a show car where you just want something different for the hell of it. The reason for twin turbos is to prevent lag, or at least any significant amount. There is no such thing as lag with a supercharger so (probably) all you would end up with is increased parasitic loss over a single larger unit like somebody already said above.