None the Wiser
I’m a two time member of the mile high club. Both times were on overnight flights during a non-busy time (when most people are sleeping). The back of most larger intercontinental capable planes (777, A380, etc) have one or two elongated lavs. Both people should be standing and facing the same way. The way it works is one person leaves their seat first and goes into the lav after identifying to the other party which lav it’s going to be.
Lock the door, but have the party lift up the ‘no smoking sign’ on the outside of the door and unlock it from the outside. Once inside, you know what to do. Make it quick and then one person leaves and lock the door again. A minute or so later, the last person leaves. Nobody had any idea both times.
Story credit: Reddit / rcooplaw