There are a good handful enough between Johto and Kanto. Let's see, Johto has.. the path in Mt. Mortar, the whirlpool island caves, the cave path that reaches between Blackthorn and Violet. Those are the ones I'm thinking of offhand. Then Kanto has the path through Mt. Moon, a cave to the north of Cerulean, another to the south that takes one to Lavender Town, and the ones scattered through the Seafoam islands.