This happened to me last year at an outstation, also on a 2 person booking and the hubby's booking was fine. This was on the return trip where the outbound was also fine. The gate agent had to call a supervisor, both were puzzled at what happened and they reinstated the reservation after 3 or 4 tries, and secured a seat, through some kind of over-ride that only the supervisor could do. They indicated that this was BA who had offloaded it, not them and seemed to give it a 'BA IT' kind of shrug so I assumed they had dealt with this before.