may I suggest an engineering book?
Just think about it, in an ideal world there are no loss and friction, and if with two turbos you can achieve a good response of the engine at low revs with a smaller one, and plenty of power at higher revs with a bigger turbo, what if we could have a dedicated turbo every, don't know, 1.000 revs?
Every turbo could be designed and dimensioned specifically for its range of revs, without bothering considering what's going on later or before of it.
It's basically what VVG is all about, changing the geometry so that it can be modified accordingly to all the conditions it has to face, so basically, in a single system, you try to have more of them.
A clever idea to gain space and efficiency in the real world.
Yes, thank you, it was both a mistranslation and a brain fart as it was late and I didn't even check, sorry.