Why do I create 2 characters when starting a new playthrough?
Apparently the first character is your main character, and the second character is "who your hero dreams of". According to one wiki:
After your character is created, you will have a chance to chose Who do you dream of? Who attracts you? This choice does not limit your romance options in-game. However, while camping at night, the illithid tadpole in your brain will use this form to communicate with you, try to convince you to finish the ceremorphisis.
gamepressure posted similar information. (Note: I have not played the game, yet).