I only see a W2 rectangle port(P5249194) in a dual plane. It's a cast iron piece and rpm range from idle to 5300. I think a dual plane with a W5 head would be a moot point unless your running a class and have no choice.
Thank you for the information. I just bought a pair of W5s and they came with a MP single plane manifold and a MP dual plane manifold and was trying to figure out why there was a dual plane for those heads (as you said, perhaps for a specific class). I am trying to find out exactly what class/purpose that is (just out of curiousity). Thanks again.