Dual dialogues allow both main characters (in singleplayer or co-op) to contribute to a decision or comment on a situation.
How do I know who'll speak first?
Day 1 Patches on PC games are ok to me, thanks for the reply =)
(Also, can there be 2 males or 2 females main characters? And can each player have an henchman? All the co-op videos I saw, Player 1 had always 2 henchmans, and player 2 had none...)