Not entirely sure of this myself, but here's what I know:
1) Sequential twin turbos - idea is, small turbo provides low down boost, then a larger turbo provides the boost higher up when the small one runs out of puff. About the only good thing to come out of this setup was the development of engines with turbo and superchargers (supercharger in place of the smaller turbo). Cars that use this setup include the Supra RZ, the Mazda RX-7 and the Subaru Legacy. Most performance modders remove the twin turbo setup, cos it sucks! The small turbos are often the same size, and (especially noted on Legacys) after only a few minor mods (exhaust, air filter, chip) - your small turbo cooks itself due to it running above it's safe speed.
OOPS - Edited cos I clicked the wrong button!
2) PROPER twin turbos - not sure the score here, but the two turbos work together - usually (I think
) one turbo feeds half the engine (like one bank on a V-8), and the other one feeds the other half. This way you get quick spool up AND good performance, as you can have a small turbo and it only has to feed 2 or 3 cylinders. Cars to use this type of setup include the Mitsubishi GTO/3000GT and the Nissan Skyline GTR.Try searching on Google for more info on these setups - it's not something I spent much time looking at, as I more or less figure it was a bad idea for a 4 cylinder once I glanced at the subject.
And even those people abandoned the idea as it was a waste of time and more importantly, their 
