I want to Boost an Electric Shower

You cannot boost the flow from an electric shower if that flow is limited by the KW output of the electric shower but you can stop the electric shower dropping out and running cold when the incoming mains pressure is low or fluctuation when others use a tap in your home.

A single SPB is all you need.

Buy a SP1 Manual Pump Now
Buy a SP2B Fully Automatic Now (fit and forget – plug and play ready)