How to Acquire
They can be found next at the Crone's Hand Saloon during the Dust District mission. They are playing in the central courtyard, but are surrounded by Howlers. The protagonist can listen in on their song from the balcony or the roof, away and safe from the Howlers.
They appear one last time in the Palace District during the mission The Grand Palace. However, if the protagonist has robbed three or more black market shops beforehand, Howlers will have instead set up an ambush in an attempt to find the person who has been robbing their stores. As it is their last appearance, the achievement is locked if the protagonist robs more than two stores. Also, listening to the pair playing their instruments without singing will not count towards the achievement.
The musical duo will not appear the second and third time if they are killed during their first encounter, thus also locking the achievement.