Why lock half of a double door?
A lot of double doors latch one door into the floor and top rail of the door frame, and the door that opens just latches to that fixed door. It’s an easy set up to install, and doesn’t come out of alignment in use frequently. Double doors in multi-story buildings are often there for emergency access issues, with the expectation being that both doors will only be open in fire situations, or perhaps times when large objects are being moved in or out of the building through those entrances. The rest of the time, only a single door is allowed to be opened, to reduce air flow into the building, and to control ground floor lobby wind due to stack effect. In tall buildings with significant stack effect, there can be a dangerous periodicity to the stack effect, that will allow both doors to be opened, only to start a massive inrush of air that accelerates up the elevator shafts, and eventually slams the doors shut with such force that they
