i think the twin turbo is to make the power smoother instead of getting all the power at once when just the one turbo kicks in. And from most of the high hp supras ive seen, they usually have a single turbo like this:
http://cgi.ebay.com/ebaymotors/Toyot...81770043QQrdZ1
im not that great with cars so... and there never was a single turbo from the factory