Once you finished the Bandit Attack story, have morale 25 or higher at the Bandit's Camp, fully completed Bareshade Expansion, helped Logan out with work at sawmil and visited the tavern at night, you will encounter a fight between Logan and Barrel.
Calm him and sleep (8hrs) at you new home in Bareshade. There is a chance for Logan to visit you, let him in and refuse to take advantage of the drunk situation, which in turn you'll get his 2nd heart.
Actually have sex with him will let him feel betrayed and used and you will get a black heart, which means he'll hate you.