Better spool up and more low down power. Twin setups tend to have smaller turbos whether they are inline or parallel. It doesn't make sense to run 2 cylinders into one turbo and then 2 into the other one though, you need a constant flow through the turbo rather than pulses.
If you want max top end power your only option really is one large turbo, there are limits on small turbos and how much they can compress the intake.