Both would be good, more cooling will always be better but you could set up the water injection system and wastegate control so that it runs through your Syvec. Then if the sytem runs dry (either a flow meter or a swith in the tank) then the ECU either keeps the HP wastegate closed so the HP does the marojity of the work keeping HP compressor inlet temperatures down and then opens both wastegates at high to avoid over boosting, essentially turning the sytem back to a single stage. It would kill the power as you'd essentially only be running one small turbo but it would safe guard the engine. Guessing Ed (Fusion) would know more on the ECU side of things.
We've been making turbos out here for about 10 years for the Chinese domestic market, I'm busy trainng engineers on the technical/aerodynamic side as they are all very green. Doing a 3 month stint (2 months down now).