Actually Mazda engineers did this on a 3 cylinder called the 13B rotory motor that is in all 93-95 RX-7s.(Sarcasim so I don't get flamed for this) The whole point is to have two different sizes so that when the exhaust is light and pushing the small turbo you have power and then higher in the RPM range you begin to spin the larger turbo so you have higher turbo output through out the RPM range. Nice idea but IMHO single is still better if you're looking for high numbers on the dyno.