Ditto the others' advice; definitely two independent discharge hoses.
Also, think about the order of precedence you want to take place and locate your bilge pump sensor switch accordingly. We have 3 bilge pumps but, if I was throttling it down to 2, I would:
Locate the electric pump's sensor quite low, put a buzzer alarm in series with the switch on the DC panel, and you'll be 'told' by the pump if water's coming in. That would be your alert to take a look and, if need be, supplement the electric with the manual pump while sourcing the problem.
Alternatively, if you want the electric as a back-up only and prefer using the manual normally, locating the sensor for the electric a bit higher (again, with a buzzer), will give you a chance for 'maintenance' pumps by hand but, if you suddenly start taking on larger quantities, the buzzer will sound and pump kick in.