Here's a schematic of how we did it. I won't bother with the electrics, I assume you are after the plumbing. We have only 3" headroom in our "bilge", and it was very problematic to install the pump & associated kit while avoiding any serious demolition work. The manual bilge pump had been installed when she was built.
First up, we found a decent "small" motor/pump/screen combi that would work flat & fit in the space. It must be self priming.
second, make sure you float switch is above the pump inlet - Idea being the pump does not burn out sucking air before the switch shuts it off. I reckon float switch setup is a major topic in itself.
Routed the two hoses in parallel, manual pump was 1" soft spiral, and the electric bilge pump we used garden hose 1/2".
We put an overbend in both hoses, then dropped them into a Y piece, then connected that to the existing outlet. The intent is the water will "drop" into the outlet, and not be able to travel back up the other pump.
We also installed a check valve (non-return valve) into the elec system, to ensure that when the manual pump is being worked, it cannot flow back into the electrical pump. This will cause a small head / flow loss. The valve fitting also had a 1/2" to 1" adaptor.
We did not put a check valve into the manual pump, as it has a none-return valve on it's outlet, it is not suscetible to debris (as an electrical pump might be), and the outlet pressure of the elec pump is low - Compared to the force with which you can pump a manual pump.
I trust this helps. All-up cost was a bit over $300, with bits from Bias, Whitworths & Bunnings. Can give you more info & photos if needed

Poods