It would be, but the problem with the dual turbo setup is that the smaller turbine will end spending most of it's life at full rpm. While it doesn't sound bad, since it's fully lubricated and such, the heat that it generates for itself will take it's toll on the smaller unit, and will end up causing the small snail to be replaced incredibly soon (or rebuilt). Nice for a race car, that gets rebuilt every so often anyway. Not for the daily driver.

Running two of the same size turbos will get the life span back up. But you still have the nominal lag associated with a turbo. You just have two turbos now.

One big spinner will give you lag, but if it's a ball bearing unit, with all the trimmings, then the lag would be back down to what the two smaller ones would be. Then you could have the massive compression of the bigger unit, reliability of one turbo and still get huge gains.

I'd go twin with two banks (V6) but single with the inline of the RB series.