I was always under the impression that "Both" of your characters are the "Main" hero, and that whoever is closest to the NPC at the time the conversation is started (in the case of auto-started dialogues) is the one who is "main" for that conversation.
Otherwise, you could simply kill off one of your characters and leave them dead... or unchain them and leave them standing in town forever..
That's usually true but when you zone or teleport into a new area and a conversation is automatically started, the game will always choose the 'main hero' and that person is the only one who can take part in the conversation. It's even explained that way in the tutorial.
