guard-lock

from The Century Dictionary.

• noun A lock placed at the junction of an artificial canal with the sea or other natural body of water whose level is subject to an extent of fluctuation undesirable for the canal. When boats are required to enter or leave the canal while a difference of level prevails between the water in the canal and outside, a pair of locks is necessary; otherwise a single lock is sufficient.
• noun Any lock used to guard a keyhole or to guard another lock, as in the locks of the boxes of a safe-deposit vault, where the key to the guard-lock may be held by the attendant of the vault and the key of the box by the renter, both keys being used to open the box. See duplex lock.