I didn't mean to say that one piston can suck gas from the other crankcase. This is of course prevented by the reed valves. I was trying to express that the piston moving at its max speed starves the other piston almost at TDC. It sort of "steals" air from the area in front of the the other carb insulator.

But now that I have done more "research" on airbox design I tend to agree that the reason for the webbing is not the above. It probably has more to do with resonance in the airbox and waves travelling between the intake tubes and the carb insulator mouths. And for this, you need two airboxes.