There's a write-up at this link:
http://www.mkiv.com/faq/faqtt.html#turbosystem1
...also more info in the NCF, starting at page 88.

thanks. that makes sense the way they do it. what stops you from upgrading the 2 turbos and ecu and using all the stock valves and things like that?

Space is at a premium. Toyota had to cast fully custom compressor and exhaust housings to fit those two oem twin turbos end-to-end within that space (that's FOUR custom housings, each very unique). Even if you had the $$$ and facilities to cast larger custom compressor & exhaust housings, it'd be tough to get anything larger than the oem twins to fit in that space. Also, you'd probably have to increase the I.D. of the exhaust manifold, piping & valves to accommodate the increased flow. In theory it could be done, but you're probably looking at a project that would cost you $25K+ in R&D. Most folks just go with a $5K single turbo kit instead...