Here's another take.
During the original certification of 727 (late 50s??), they have to have so much "window" area in the cockpit and to widen the existing cockpit would have weaken the structure so they put the little windows in. Hence the "combined" window areas would meet and pass the certification requirements. They did away with that when 747 came around.
Heard it from some engineer a long time ago so don't quote me on it.
Probably the same folks that put the stupid little mirror in the 747 cockpit so that you can see the flap position from the left seat, albeit upside down.
