I suppose a twin turbo with the same power output with a single turbo will have more resistance, but the engine will have the same duration ...
I was talking about the new GTM twin supercharger system (370Z only I believe), superchargers differ from turbos - a turbo is driven by the exhaust where as a SC is driven by the engine itself so you lose power when not on boost. I would have thought having twin SCs instead of a single would increase the amount of BHP lost.