You could do it if you found a Gated-Wye. The odds of you finding someone who makes a 3/4" Y with a built in clapper valve is pretty much nill. You could invent one.
Two bilges to the same outlet would increase the PSI, so you would flow more GPM. However, it would not be the same as 2 separate lines. What's wrong with just running a larger bilge, like a 800 or 1000?
Unless you own a X2, or have a ski with no hood seal what-so-ever, a single 500 should handle any condition. I ran a single 500 at Daytona this year with a very very bad hood seal and still had very few problems.