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I always thought that twin-turbos were two turbos of same size, which spooled up at the same rate and rpm, and bi-turbos two turbos of different sizes that spooled up at different rpms. For example, on the bi-turbo, one turbo would kick in earlier at the lower rpm and the other at a higher rpm to eliminate turbo lag. In order to make engines more efficient and to eliminate turbo lag, the Japanese had engines with both supercharger and turbocharger; the supercharger activates at lower rpms and the turbos would kick in at higher rpms ensuring constant power range.
I could be wrong with the whole bi/twin theory, but a supercharged/turbocharged engine does sound interesting. You would need a lot of room, and the N55 535i has a lot of room left and right side in the engine bay compared to no room at all in the N54 335i engine bay.
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