These characters are both in the Lust section of the game. Francesca is right when you drop down from the first section with the swinging ropes. Later in the level when you are on the Tower of Lust, right after you burn Cleopatra with the snake heads that spew fire the platform will stop. Forward and slightly to your right you will see a pole. Head between the pole and the wall and the camera will shift and you can climb down to absolve Paolo. Once you have absolved both you will get the achievement.