The document discusses why running two pumps in parallel does not necessarily result in twice the flow. It explains that parallel pumping is best on a flat system head curve but usually provides little benefit on a steep curve. Series and parallel-series pumping are presented as alternatives that can improve flow in some situations by keeping pumps within their performance ranges. Parallel-series pumping allows each pump to handle average daily flows, but converts one pump to series operation for higher flows. This arrangement provides benefits over simple parallel or series configurations. Examples are provided to illustrate different pumping scenarios.