What does this double lock do?
Well, the ones inside a bank's vault need Their key and Your Key to open Your safe deposit box. For instance.

Designed that way.

Requires two keys to operate. Normally applied where two separate people hold one key each and need to both be present when the lock is opened.

Depending on design, could also be used so that a primary key is used at the start of working hours and then a secondary key will operate the lock, but only during working hours (the primary key would be run around the building and used to lock the primary lock at closing time.)