There are two people A and B, both are geographically apart from each other and can't meet. They exchange messages through a Box which has two latches

There are two people A and B, both are geographically apart from each other and can't meet. They exchange messages through a Box which has two latches and the box is delivered by a postman. Given the chance the postman will break into the box. The box can't be opened if there is at least one locked latch. Both A and B buy a lock, both get two keys with them (A has lock1 and B has lock2). Now how can they communicate securely using this system?
Suppose A has lockA and keyA, b has lockB and keyB
A put the message in box and lock the box using lockA send to B
B lock the box using lockB and send to A
A use keyA to unlock lockA, send to B
B use keyB to unlock lockB and read the message