You should be able to just select which ever character you want to do the talking and use them to talk to the NPC's
Both characters so to speak are the main characters, If you are talking about how the portraits in the top left are you can click and drag these to change the position if you require.
But when talking just make sure you use the one with charisma, you can also tab through your characters by using F1 and F2 etc (F3 and F4 if you have henchman to)
