twin turbo = half the exahust to spool it.
which means a dinky turbo takes forever to spool, and in the end, you're topped out long before you would be with similar lag from a single larger turbo.
not to mention all the extra weight, and double everything in terms of cost. (2 waste gates, 2 downpipes, 2 sets of charge pipes to the ic, a custom IC with 2 ins, one out, etc etc etc.
its a waste of cash, is slower, and a bitch and a 1/4 to get working right.
thats why all the supra guys go to single turbo when they build.