Fast glass biplanes

Imagine that you're an air molecule; how do you know if
you're 5' or 10' along the wing? You don't, when the wing comes
along, you just move along the bottom or zip across the top.

Those molecules are smarter than you might expect. G

There can be significant spanwise flow of the air. Like most things in nature
air finds the path of least resistance and sometimes this is not where it was
headed when the wing bounced into it.

Even if you take the same 40 ft high aspect ratio wing, saw it into 2 halves
and manage to attach it to the fuselage with no increase in interference drag
it's going to be less efficient than the 1 long wing - because of the spanwise
flow. Winglets help, flow fences help, joined wing tips help, elliptical
planform helps.

Look up W. Kaspar and his work on tip vortices.