A regulated two-stage turbocharger system is described, including high-pressure and low-pressure turbocharger units. The low-pressure turbocharger unit includes a twin volute turbine portion. The turbocharger system includes a valve system having valve members that are independently controllable so as to selectively control an exhaust gas flow into the twin volutes of the turbine portion of the low-pressure turbocharger unit. By-pass valve systems are also provided for the twin volute turbine portion of the low-pressure turbocharger unit and the compressor portion of the high-pressure turbocharger unit.