Not to resurrect an old thread, but I thought:
Bi-Turbo - 1 small, 1 big turbo; the small turbo spins up first to help get the larger turbo going
Twin-Turbo - 2 identical turbos, splitting the work in half (1 turbo: 3 cylinders for the 335i) to get it going
Man... I can't wait until end of Dec for my 335i... I feel like Cartman on last night's South Park (where he waits for the Nintendo Wii)!