In Jails they're called "Sally Ports". The whole concept is you never have a door open to the out side world. We used them when we'd go from the secure to non secure area. One door opens, you step in, that door closes, the next one opens. They were controlled by an operator in a remote control center with cameras at every door. The doors are air operated and slid like a big pocket door. We also had them outside (rolling chain link gates) for vehicle entrances as well. Any way, it would be easily applied here. It would probably be the easiest most secure way of doing it. Even keeping them both closed during normal ops, if there is room. That way pax would never know when the crew door was opened. If you made it a sliding pocket door it wouldn't even take that much space, maybe 6-8" at the most. When I left the jail for XJT I always thought of this stuff when we'd have a pee break.