The stock twins are old, old, old technology. They'll do the job but not very efficiently. My reading has indicated that the stock JZ (both 1 and 2) build up heat between the rear turbo and the firewall which typically causes the rear turbo (aka turbo #2), to fail before #1.
Yes you could build up an aftermarket set of twins, and many people have and still do, and they're awesome. But it's the center section of a turbo that is expensive, so buying two small turbos is twice the price of one large one. Also, there's twice as many oil lines, twice as many coolant lines... boost control becomes more complicated (rocket science complicated if you want sequential twins). Also, twins require a complicated exhaust manifold (sometimes required to be built custom for an application), crazy intercooler piping, umung other things.
A big single is simple and straightforward. It's not as flashy to say as "twin turbo", but it gets the job done for ALOT less money. A well designed big single will have arguably less lag and comparable boost threshold as a similair horsepower parallel twin setup, with less than half the complexity and much less than half the cost. You may think "well Don, the only difference is another turbo! How could it be less than half the cost?"... a good point:
Two wastegates (those aren't cheap), way more couplers, hoses, lines, and connectors for the oil system and coolant system. More piping and more complex welds to hold the piping together, alot more hours put into designing the intake system to fit under the hood... time is money. Also, the downpipes for a twins set is probably going to cost you a chunk of change... Alot more than it would have for a simple single pipe.
The upside to twins vs single? There isn't one compared to parallel twins. New turbo technology has made it more efficient to spin one big turbine than two smaller ones with the same amount of gas to make the same horsepower. However, twin sequential is fucking cool, and would kick the crap out of a comparable hp single or parallel twin.
