So I discovered this on accident today and can't figure out a way around it short of starting a new game.
When playing through, on multiple accounts, be aware that the quest giver related achievements will only be granted to the person who turns in the final quest to that particular character.
For example, Player 2 turned in Tree Trunks final quest while we were playing together today. She got the Tree Trunks achievement, I did not. When I tried to talk to Tree Trunks, she thanked me for all my help. I attempted to reset the game without saving, no such luck.
Just a warning for people playing on multiple accounts.