If both systems are of the same quality and sized for similar top end flows twins will generaly spool quicker than singles for a couple reasons.
1. Turbos get their energy to accelerate (spool up) from the pulses in an exhaust system rather than just mass flow, by having two seprate systems you get greater pulsing, this is why on single turbo setups a "spilt pulse" turbo will spool faster, especialy if plumbed so that each consecutive firing cylnder goes to diferent side of split housing.
2. Two small turbos with the same flow rating as a larger single have less mass to accelerate (not as influencing to spool up as above)
Spool up speeds in any system can be generaly increased by running a decent electronic boost control rather than a bleed system.
Being a Vee engine will be much easier to plumb up a decent twin turbo system than trying to get reasonably equal runner lengths to a single turbo system.
regards
jon
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