How about doubling the number of entrances/exits on a plane: have one on the front that boards the first half of the plane and one on the back that boards the rear half of the plane? I'm sure there would be some way to "thread" passengers once they all go past ticketcheck, perhaps with a re-engineered variant of something like: Rows 1-20 to the left and Rows 21-40 to the right. Only potential families sitting in rows 20-21 would have to be separated at boarding time, but they'd meet right back up in the middle of the airplane.