In order for a pump system to be effective it has to be able to provide the required pressure, aka head, at the required flow rate. Any upgrade has to be able to handle both of these otherwise it is not adequate.
A series setup is typically used to boost the pressure of the system without specific regard to increasing flow - not that it can't be used that way though. Parallel setups are used to increase flow. For a given increase in flow, or a "surge," a series setup will typically lose more pressure than a parallel setup. Now if it can lose pressure but still meet requirements, then no big deal. But there are also efficiency curves and all that to take into account as well.
That's my two cents from some basic courses in water sanitation and pump selection.
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