WHY VOD!!
Im sure there must be some great reason considering some smart Subaru engineers couldn't get it right, but if Mazda could do it then why couldn't Subaru.

This is taken from the Mazda website :

A major downfall of other sequential twin turbo systems is the transition from single to double operation. This is due to the second turbo not spinning fast enough when its called on, resulting in the engine "coughing" momentarily.

Mazda solved this problem by spooling the second turbo to a pre-boost speed of around 100,000 rpm with exhaust gas circulation. When the time comes for the second turbo to cut in, a by-pass valve is shut to "surge" the compressor, spinning it to over 140,000 rpm. This then assures a smooth coupling with the primary turbo. Once at the required speed, it receives a full share of exhaust flow to add its boost with the primary turbo.