I would think that most will suggest that the best practice is to have two totally separate pumps,- manual and electric; however, it's not uncommon to find one effluent through-hull from two sources. With a "Y" or "T" merging the outflow just before the through-hull each hose should have it's own vented loop. I think the vented loop preventing seawater from entering is a better choice than a one-way valve.
If you happened to be constrained to use a single outlet and the pumps will not run simultaneously, you could consider using a normal valve (as opposed to check valve) in the elec pump line.
You'd normally leave that one open, only closing it when you use the manual pump on its own. The elec bilge pump running will not push water into the manual pump because the latter has a one-way exit valve.
........................ you could consider using a normal valve (as opposed to check valve)......................
By "normal valve" do you mean a Y-valve with a lever that selects one passage or the other? If that is the case I think it would be suitable and an excellent suggestion, but the same action can be accomplished with a passive system where both effluent hoses are left open and each with their own vented loop.