You were born as soul twins, twins that share a single soul and act as an extension to one another. These bodies share part of their knowledge, which includes their class training, their proficiency trainings and other knowledge like languages. How you became tied to eachother is up to you, the player. You could have been born like that, or some magic could have made your body like this, or maybe some curse split off your body into two. Decide that when you make your character

Skill Proficiencies: Two of your choice

Languages:      Choose one of your choice from each category, or two of your choice from a single category.

Equipment:       A leather-bound diary, a bottle of ink, an ink pen, a set of traveler's clothes for each clone, one trinket of special significance, and a pouch containing 10 gp.