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.
Right, easy solution to BOTH things, to make everyone happy:
1. When one player starts a dialog, the 2nd players is given the option "Watch Dialog? Yes / No"
2. If Yes is selected, they proceed to see the dialog as if they had initiated it. If No is selected, they can continue playing as they currently do now in the game.
This isnt rocket science, nor is it a functionality that is hard to program. Its something that just wasnt concidered, or had time allocated to doing it.