The pump curves below give the results achieved with a single SP2B ShowerPowerBooster pump. Just double the pressure for 2 pumps in series (the SP22S)
The output from a pump is always a compromise and a balance between pressure and flow provided by the electrical energy you put into the pump.
If a fixed amount of energy is imparted into the water it can increase pressure and it can increase flow and for every application there is an optimum pressure/flow ratio.
In order to give a strong spray pattern a WRightChoice Shower head combined with a single SPB will lose 2.3 metres head in the shower head to create a fast/strong spay pattern and deliver 5.9 litres of water. To get 6 litres of water you normally mix 4 litres of hot water with 2 litres of cold so please use the boost at 4 litres head to calculate the hot boost and not the 6 litre total flow.
Remember that showers mix hot and cold and your shower head may not be as good as ours. You may also have a mixer that is designed for Central European high pressure systems rather than one designed for the lower pressures we can expect in the UK.
If you do have problems you can double boost the hot with a SP21S or balance flow with a SP22S.