Pumps in series double the head at the same flow condition point. One pump discharge is piped into the suction of the second pump producing twice the head capability of each pump separately. The second pump however must be capable of operating at the higher suction pressure which is produced by pump number one.
This mode of operation is a very cost effective way of overcoming high discharge heads when the flow requirement remains the same. As a matter of fact that is exactly how multi-stage pumps, such as vertical turbine types operate.