If you put a 5lb spring in both wastegates on a greddy kit, you end up with 5psi in the manifold (subtract a little for intercooler looses).
I think the question are you asking has more to do with the MASS of airflow, rather than pressure. Remember that turbos do not create pressure, per se....they create a mass of airflow. Pressure can vary based on ambient conditions, but the key measure is how much airflow the turbos are creating.
For instance, lets make an assumption that 30lb/min of air is needed to create 10psi of pressure in the manfiold. In this example, each turbo is supplying rouhgly 15lb/min worth of air. Each turbo is contributing half of the airflow into the engine.
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