It's called staged sequential turbocharging. It's most commonly used in piston driven aircraft engines and diesel racing engines. There are several issues that prevent these from being used in standard combustion engines. 1) Turbo lag is exacerbated with each stage you add. Aircraft and diesel engines don't change RPMs as fast or as far as petrol engines so lag is less of an issue. 2) Petrol engines can suffer from pre-ignition issues where the higher pressures cause the fuel/air mixture to detonate before the spark plug fires. Obviously this is bad. More common, and effective, are parallel or standard sequential twin turbos. Parallel allows you to use smaller turbines which spool faster and reduce lag. Sequential systems use a small turbine for low RPMs and then switch over to a larger fan for the high RPMs. This reduces lag while maintaining high flow/boost at higher RPMs.