Scylla and Charybdis

Scylla and Charybdis are two sea monsters of Greek mythology who were situated on opposite sides of the Strait of Messina between Sicily and Calabria, in Italy. They were located in close enough proximity to each other that they posed an inescapable threat to passing sailors; avoiding Charybdis meant passing too closely to Scylla and vice versa.

Scylla and Charbydis in "The Horse of Bronze"
Cheiron's expedition across the Inner Sea managed to avoid Scylla and Charbydis both in leaving the sea for Ocean the Great and returning to it.