Ah, OK, so the issue may be head available to the upstairs shower: if you shut off the Water to the hydro generator before using the shower then you should have ~2.8 bar at the upstairs shower. Fine.
But with the hydro gen running at max power (limited by the capacity of the 63mm penstock), pressure at the bottom of the penstock is only 2bar, and when you turn the shower on with the penstock already running at capacity (4.1L/s @ 2bar in my powerspout result) the pressure will drop further and you might find your shower is disappointing as well as losing output from the generator. It may still be OK as long as the shower flow rate is significantly less than the 4.1l/s the generator is using.