It seems likely to me that the sill is designed to resist pressure principally from inside the plane to outside the plane. If sea level pressure was trapped be

It seems likely to me that the sill is designed to resist pressure principally from inside the plane to outside the plane. If sea level pressure was trapped between the two windows you'd have pressure in the opposite direction on the inside window whenever the cabin altitude increased, which would happen every flight and induce cyclic stress on the sill that it wasn't designed for.