Hmm interesting question. Without searching/researching the intrawebz, I imagine you add the boost pressure feeding the turbo to 14.7 (atmosphere). Since the P1C is higher, the pressure ratio drops, placing you lower on the compressor map. Subsequently, you can up the boost pressure of the second turbo to bring P2C higher, and bringing your pressure ratio back up. So I would imagine that running sequential turbos allows the second turbo to be run at a much higher boost pressure without operating off the compressor map.