Chatterbox: Down to Earth

This Person (TP) can only say "Hello, how are you" when Someone Else (SE) says "I am from London." SE can only say 'I am from London" when TP says "hello, how are you." TP can only say "Hello how are you." SE can only say "I am from London." They must respond to each other, but they can't speak until the other person speaks.

They are in a enclosed room in which no one can get out and no one can get in. SE and TP were in room while it was being build. The cannot leave the room.

After hours of silence, they finally say what they can only say in responce to each other. It continues for a few hours until they go silent. How is this possible?