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Just packaging really, there's a fair bit of space either side of the mini engine but not much in front or behind. 2 small turbos would mean 2 pipes from the rear facing exhaust ports, merging before the turbo flange, through turbo then one pipe on each side to tiny intercoolers and then into the plenum.
Especially with a 16v head there's even less space front and back, a 16v head with twin webers won't fit under the stock bonnet with any sort of velocity stack. Assuming throttle bodies take up less space by doing it this way I'd be able to use slimmer pipes and give it a greater chance of not protruding.
This is a single turbo 16v setup.
Why would you use two turbos for that? Single would be the way to go