I'm with Terra Nova, avoid check valves in bilge pump discharge lines and don't share a discharge line.
I think people are too afraid to put holes in their boat. If it is done correctly, there is almost no chance of a problem. In your case you're looking at a 3/4" thru-hull for a small bilge pump. Mount the thru-hull where water
can't flow back in even heeled (transom?) or if you can't do that, put a vented loop in the hose higher than the heeled waterline.
I'd drill a pilot hole through from either direction where you want the thru-hull, then use a hole saw to cut part way through from the outside, then the rest of the way from the inside. This just makes a little neater hole.
There is no rule
that says you can't install a seacock on a bilge discharge if it worries you.