I suggested awhile back that the person who didn't initiate the conversation should be able to see the choices available to the first player, but be unable to select it.
What could be a lot of fun is to be able to both talk to the character at once - so you have two source hunters engaging in dialog (and choosing dialog options) with the NPC kind of like how it would go in real life. You wouldn't have one person have a 10 minute conversation with the second person waiting silently on the side.
