How did the shepherd and the messenger from Corinth know each other?

When the Messenger arrives with news that Oedipus's adopted father, Polybus, has died, he also gives the news that Oedipus was adopted. He tells the story, which is that a baby was given to him as a gift for the King and Queen, who then raised him. The baby was Oedipus, and the giver was the Shepherd. The reason the Shepherd had the baby was that Laius had ordered the baby killed in order to present the oracle's prediction from coming true. The Shepherd and Messenger have only met this once, but what a meeting!

CORRECTION: The Messenger had been a shepherd too, and they'd been around each other tending sheep together for several years at the time the Shepherd gave the Messenger Oedipus as a babe. That's revealed in just one small speech, but I just verified it.