Seems like they could save time if they could open the back door and send all the extra carry-ons out to the cargo hold that way -- no more sending them the "wrong way" against the incoming pax. I'm sure it's not that simple to open a second door, but I have heard about a new semi-automated jetway system that they're supposed to be trying out soon to do two-door (front & rear) loading and unloading. It sounded expensive, but the company pitching it to United seemed to have convinced them that it might shave off enough gate time for each flight that it would pay for itself over time.