Compound charging is big in the diesel world if you want a torque monster and has beenaround for years. Look for Mike Ryan's Pikes Peak truck on YouTube. Land Rover TDV6 are sequential in operation. I don't know what sort of response you'd get in the VR38 though.
sounds like it has a much higher moment of inertia and twice the drag of a stock turbine design.
Only way round this would be to selectively decouple one set of fans from the rotating assembly either by a clutch or a one way bearing. Nice idea but in practice introduces massive of potentially mechanically unreliable components into one of the nastier environments - upwards of 1000°C and 1000 rp/second!
Makes more sense to compound turbo, or better yet, the F1 idea of using the turbo to directly drive a motor instead of a wastegate, and maybe store charge to preaccelerate the turbo when needed.