Pathfinder already solved this problem, any time there is a skill check in dialogue it automatically uses the skill check of the character in the party with the highest bonus, no matter who is "talking", and mousing over the selection will show you whose skill check you're using. It's pretty seamless and it works because the party engages in conversations as a unit, just like a real tabletop party does. You usually have a main character that's driving the specific conversation but party members interject or argue frequently.