A single turbo is only efficient through a limited pressure rise. In a performance diesel it is more about pressure than flow. Airflow is much more consistent in a diesel as it is strictly dependent on rpm and independent of load. You have a smaller variation in flow as well. The Cummins has a sixfold increase in flow from around 500 rpm idle to 3000 rpm for a race pump. Some go 4000 but they probably idle a little higher.
Your typical gas engine likely has a tenfold flow variation or better. Point of this is on a diesel your turbo[s] operate over a higher range of pressures but a lower range of flow. Gassers have a lower range of pressure but a higher range of flow.