ORIGINAL: da Rock

Ever looked hard at the airflow diagrams in the "how does lift happen" texts?

There is a significant upflow ahead of a wing. (Just as there is a significant downwash aft.) It has been theorised that biplane wing most certainly have upflows and that the upflow of the lower wing influences the flow that goes to the upper wing.

It would follow that since the upflow of any wing affects the airflow, artificially influencing the upper wing's performance might result in conditions where rigging that upper wing differently from the lower one would have beneficial results. Or not-beneficial ones. And rigging with no decalage might have unwanted consequences or beneficial ones.

It is already well understood that the distance between those two wings really matters. It's just not usually stated what those things that matter are.