I for one absolutely do not want the "same dialog window" solution many seem to. The second party should not be forced into the dialog if they don't choose to open it.
The primary advancement this game had made in turn based party based RPGs is to break the "follow the leader" standard which reduces every player other than the "leader" to an observer. My friend and I only play Divinity together and usually do our own thing in town until somebody says "hey, you should listen to this conversation", at which point the other opens the journal and follows along.
Yes, this system needs improvement so as to be smoother. A simple on screen notification that the second player cab click on or not would be best. But to force the old standard of every party member being handcuffed to the " talker" is a step backwards.
