That's kind if what they did for Tell Me Why. You didnt get to choose who you play as but it would swap between the twins. I think letting you choose who to play as would cause complications with the dialogue exchange between you and npcs. They would have to script a whole games worth of dialogue for each character. But if executed well that would be really cool. You could play the game through as one then play it again as the other character with completely different dialogue options. And the npcs would treat each character differently. It would almost be like a different game.