Odysseus talks to his men about how to avoid Charybdis, but he does not tell them about Scylla. He gives them a "pep talk" and reminds them how they were able to escape from the Cyclops.
He knows that if they get too close to the whirlpool he could lose the whole ship, so he wants the men to put all their effort into staying out of Charybdis. Unfortunately, in order to stay out of the whirlpool, they will have to sail close to the monster Scylla, which means that a few of them will be eaten by Scylla. Odysseus knows if he tells his men about Scylla, they will be overcome with fear and will not be able to navigate past Charybdis, so he only tells them about the whirlpool.
Circe sent Odysseus to meet with Tiresias, a prophet from the house of death who would provide him with a glimpse into his future and the challenges that would face him and how to overcome them. He also got an opportunity to speak to the dead, such as Achilles and his mother, while there. Odysseus later went back to Circe who promised to chart a safe path for him and his crew to ensure they reach home safely. Circe, in private, prepared Odysseus for his trip through the Island of the Sirens, then between Scylla and Charybdis. Circe warned Odysseus that Scylla had six heads and each devours sailors passing through while Charybdis is a whirlpool that would swallow and wreck the entire ship. Circe advised Odysseus to follow the path closer to Scylla who would devour six of his men rather that Charybdis who would devour them all. Odysseus did not mention Scylla to his crew but instructed them to avoid Charybdis to give them hope of surviving, but he knew he would lose six of his men to Scylla. To maintain their courage he kept information about Scylla from them and reminded them of his skills that saved them from the Cyclops.