It's a 'dual volute' casing.

Two separate paths to discharge provide more balanced forces, allowing for a lower minimum flow.

A volute in a centrifugal pump is a chamber that starts out with a smaller area and gradually increases the area. This slows the velocity of the fluid and converts that velocity into pressure. In a single volute pump, the changing velocity and changing pressure around the circumference of the impeller results in a net side load on the impeller and shaft. By using a double volute, the two volutes each produce a net force on the impeller that are opposite and tend to cancel each other out. A lower net side load on the impeller results in lower deflection and smoother operation. Since the magnitude of these unbalance forces depends on the pressure, which depends on the operating point on the curve, the use of a double volute can allow for the operation of the pump further from the best efficiency point as TenPenny already noted.

Johnny Pellin