I don't have a link to the posting. You have to search the RC for it.
To come to think of it, the 2nd drain has to be the higher one. The regular drain with the ball valve below must be the one working submerged. When it clogs up, the water gets high in the overlfow and hits the 2nd drain which functions as a primary drain then.
Run this by speakeraddict. He should be able to make sense out of this. The key is to balance the drain and the return perfectly using the ball valve to keep it silent.
Tomoko