To get the second dusk stone fly to the pokemon league building and go in victory road and go up a few hills.At the top of a hill there will be a door go in and it will be all foggy.use defog and you will see a girl named marly she will team up with you.

Note:you must beat the game
2nd Note:she has a arcanine
3rd Note:there are only two dusk stones in the whole game!