Well, Porsche is one I can think of that went to a two turbo set up on their race cars.
The reason that they did that was they were looking to reduce the time it takes for the driver to hit the throttle and the turbos respond, i.e., reduce the "turbo lag".
They went to two smaller turbos rather then one larger one. How much advantage that gives, I can't say but in a street car, driveability is an important consideration.
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