"Y" connection by itself will allow backflow and defeat both pumps. Check valves will create too much head pressure, unless you got a check valve with REALLY light cracking pressure. Even if the manual pump has a rubber check value (which it probably does), pumping the manual one will still cause back flow through the electric one. That is unless both hoses have a considerable "riser" section.
I advise that you install a dedicated thru-hull for each bilge pump.
Last edited by jr438234606; 06-25-2008 at 02:41 PM.