While doing the human noble origin, you can choose to bed either Iona or Dairren. Making plans with one will cause the other to refuse. After the night is over, both quests, "Sweet Iona" and "Sweet Dairren" will be logged in the "completed quests" section of your journal.
Follow Iona and Dairren to the study after talking to your mother. Talk to either Iona or Dairren to eventually bed them. You have to be direct in your choices.
Either Iona or Darrien joins you in bed that night.