Most of the small pumps give gpm at 0 psig. for example for a par max plus pump has a 6 gpm rated capacity at 0 psig., At 30 psig it flows 5 gpm and at 50 psig it flows 4 gpm. Flow and pressure vary along the flow curve, more flow less head
I expect with a 70 foot boat that the pressure drops in the pipe/hose will add up.
With two pumps in parallel, the OP will want to install valves up and down stream of each pump for servicing. A large accumulator would help the OP, but if two people shower
at the same time, water
will run out quickly.
The other thing the OP could do is rebuild/replace the original pump and install a pressure switch on the discharge of the pump to kick off the pump when pressures drop below 5 psig. That would prevent the pump from burning out. Of course you would need to install a bypass switch for the initial prime and pressurization, but that would save a pricey pump rebuild