You can't fit sequential turbos to a GTR, the engine runs as two 3 cylinder engines and has 2 separate 3 port exhaust manifolds (2 afms etc).
Sequential turbos are a bad idea in general, as the plumbing is a nightmare and one turbo stalls the other if set up badly. As Gonthrax says the top-end power is also limited by the smaller turbo.
Ever seen a powerful Supra with sequentials.....? (6 cylinder engine and they came with sequential turbos as stock) Answer no - they all use big twins (2530,2835,3037) or huge single.