A turbine assembly for an exhaust gas turbocharger has separate first and second volutes that are sequentially activated via a valve that receives exhaust gases from an engine. In a first position of the valve, only the first volute receives exhaust gas; in a second position, both volutes receive exhaust gases. In a third position, a bypass passage is also opened so that some exhaust gas bypasses the turbine wheel. Unlike conventional twin-scroll turbines, each volute receives exhaust gases from all engine cylinders, and the first volute feeds gas into the B-width portion of the wheel, while the second volute feeds gas into the wheel after the contour portion.