I think you're right that both escalators will be going in the direction of the most traffic at any given time...outliers can take the stairs or the elevator. Isn't that what the Minskoff and the Gershwin do?
I wonder if the thinking about two small escalators versus one larger one is that, if one escalator goes out of service at the wrong moment, they still have the other as backup? What are the chances of two escalators malfunctioning at the same time?
