I'll be starting a coop playthrough next month, I have the same question!! I wonder how questing and dialogues work in a coop enviroment, if they're any different if the party is playing split screen or with separate PCs...
When I see your question, the first thing that comes to mind is whether or not you can chain two human controlled characters. Chaining them should result in a single quest log and a single dialogue check for certain events. If chaining is not possible at all, I have to wonder how questing in its entirety works... I've never seen a game where you can play the coop campaign with friends (like, what are those exactly?)
