I think the only good reason to go twin would be if you can't get a single big turbo to fit in you car, or you can't find a big enough turbo to match your needs. Unlikely!
The following decrease spool time:
1)Ceramic ball bearing turbo
2)Twin scroll turbo
3)Smaller exhaust turbine housing
4)Smaller turbine trim wheel
5)Smaller compressor housing
6)Smaller compressor trim
7) To some extent (trial and error), smaller diameter turbo manifold diameter piping
All of the above are at the sacrifice of high end horsepower.
So really, whether it is single or twins, or triples, really what you want to decide is where you want your power. I went with a smaller turbo for street use, because it builds boost early. I don't expect to hang at 7krpm with a supercar, but in autocross point to point speed it's pretty good (I'm a terrible driver, but that's another story).
I think what the guys are trying to tell you is that turbos are scaleable. Big twins will act like a massive single, middle twins will act like a single big single, tiny twins will act like a middle size single. There is little gain in real effectiveness, but a massive gain in complexity given the plumbing, etc.
Just totally not worth it!!
Incidentally, I just wanted about 5psi. After installing my 'small' turbo, I have it cranked to near 10psi and it pulls like a freight train. It's sometimes difficult to properly estimate the airflow you will require to satisfy the engine's requirements. In my case, I overestimated my engine's req., so my 'smallish' turbo ended up being a good compromise for both spool and top end. It's a twin scroll mitsu, and spools very quickly!
The best page I've seen for engineering info on turbos is this:www.my3kgt.insel.de/gt-file...turbochargers.pdf
Hope it helps.