This achievement is unlocked during the level up process when you unlock a destiny consisting of two classes. You must have at least 6 points in each of the two classes for a two-class destiny to unlock. When you qualify, you'll see the destiny available to you. You need not select and equip it for the achievement to unlock. The only requirement for the achievement to unlock is that you see a new entry for a two-class hybrid destiny. These are the lowest level two-class hybrid destinies available to you:
Finesse 6 Sorcery 6: Disciple Destiny
Might 6 Sorcery 6: Guardian Destiny - As shown in the below video
Might 6 Finesse 6: Duelist Destiny
The below video shows the player character qualifying for the Guardian Destiny but the one class Brawler destiny is equipped instead. The achievement unlocks: