I'd guess Stardock has a bunch of pairings where you get one or the other
Indeed. However, considering that a lot of the side quests are non-linear, it is often frustrating because you don't realise that the mage you just met will prevent someone else from joining.
I will allow myself to suggest a more fun dialog system.
Premise: A and B are mutually exclusive. You pick up A first, then stumble into B at a later time.
B: "Arrgh, I see you are travelling with A, I don't associate with those unwholesome types. It is me or him."
You: "Yeah, I never liked that guy anyway. You are in!"
A: "Screw you guys, you are mean!" (exits left stage, with his gear in tow; some may chose to fight you rather than leave quietly)
You: "A is a friend and I stick by my friends."
B: "Fine, your loss."
You: "You are the one that's unwholesome."
B: "Heretic! You die now!" (fight)