Full disclosure: This question stems from a game in which I am the GM and Bellona is a player. However, I’m going to answer this question as I would should it not involve me, since topics like this are for the benefit of casual viewers as much as those directly involved.
I am unaware of any rules regarding this sort of situation, though I think Deception is the closest match.
I handle situations like this with a normal Deception check (Adversary doesn’t apply). Reasons why:
If you are telling the truth:
You’re trying to convince somebody of something of which they likely have minimal or no knowledge, or else conflicting knowledge. Thus, you have to be convincing enough that they believe you without further evidence or anything backing you up. If you do have something backing you up, it should add Boost.
If you are lying:
You’re trying to convince somebody of something of which they likely have minimal or no knowledge, or else conflicting knowledge. Thus, you have to be convincing enough that they believe you without further evidence or anything backing you up. If you do have something backing you up, it should add Boost.
Adding a Boost or downgrading the difficulty because, when telling the truth, your story should have no holes, is reasonable, though I disagree with the blanket application of this since being truthful doesn’t guarantee that you’ll be correct or remember everything exactly.
Additional reasons why I think Deception makes sense, and why a high Discipline/Nobody’s Fool makes it harder to convince them:
- Someone who is unrelentingly skeptical (hah, see what I did there?) and refuses to be fooled by someone is going to be more hesitant to believe the truth, especially when he’s already suspicious. If a check needs to be rolled for either the truth or a lie, it means that the target is already inclined to disbelieve the acting character, and likely already has conflicting information (whether falsified or not).
- Generally, we’re more likely to believe the first thing we hear. If we then hear something contradictory, we’ll be inclined to disbelieve the contradictory information (see confirmation bias).
- Part of a high Discipline and resistance to untruths is a stubborn refusal to be taken in by something you think is false. This can make it difficult to convince somebody who already thinks you are lying.
- Somebody can say something true, but either have such little credibility or say it in such a way that you think he’s lying, even when what he’s saying is totally true. This is where a high Deception skill comes into play. If you’re really good at convincing people that a lie is true, you’ll also be very good at convincing people that a truth is true, unless they already know you’re a liar in which case you’d add a bunch of Setbacks, upgrades, or difficulty dice to the check.
Who should roll the active portion of the check? I (obviously) say the truth teller. It’s incumbent upon him to convince the hearer of the veracity of his statement, just as it is incumbent upon the liar to convince the hearer of the veracity of his statement.
In the case of someone who is simultaneously being lied to by two characters (meaning he’s being introduced to both arguments at the same time, and has not already been convinced of one or the other and has to sort out the truth), then it should be a competitive check opposed by the target’s Discipline.
Here's what it was before competitive check, as that option completely slipped my mind.
I’d say that the listener should make the check, with a difficulty determined by the skill disparity between the liar and the truth-teller. If it’s about the same, Average. If the liar is much better, Hard or more (upgraded once per point of skill over the other). If the truth-teller is much better, Easy (just Easy, no positive upgrades or further difficulty reductions).
So if the liar has 4 Cunning and 3 Deception and the truth-teller has 2 Cunning and 1 Deception, I’d set it at Daunting with 2 upgrades and appropriate Setback or Boost, with success allowing the acting character to determine the truth.