Firstly, I have been concerning myself with wether a particular claim is credible, not wether a particular person is credible. If you don’t see a difference there the only way I can think to explain it is that you can find a claim to be false, and still find the person to be credible.
Now, when I assume that two people are equally credible, and that they are not (neither of them) 100% credible, then what does that tell me? It tells me that they might lie, or be mistaken, or whatever. Either of them. There is also no reason whatsoever for me to assume that wether one of them lies in one particular instance has any effect on wether the other of them is lying at the same time. Correct?
So. We have two people who are each equally credible, and can lie; however, merely looking at the person doesn’t tell me wether they are lying. This means it’s not like “You have scuzzy hair and a bad necktie, so I doubt your credibility,” or like “You’re a whitey and I’m a racist, so I doubt your credibility.” Each person in the scenario is intrinsically trustable, based on their own personal attributes. Like identical twins; I can’t tell them apart even.
And then one of them tells me that he likes the taste of chicken, and the other one tells me he’s actually President Abraham Lincoln, brought to life again my the dark magic of Ra, as used by a japanese ex-wiccan with a drug problem…
It’s the claim itself that is less credible, not the person.
Secondly, I would like to point out that finding a claim to be incredible is not equal to automatically assuming it’s false. It’s just unlikely. If you’re going to put words in my mouth, the words would more likely be “What, he won the lottery? Him right there? Are you kidding?”, not your work of fiction.
And thirdly, as an aside, I would also like to point out that any scenario that has me actually conversing with the claimants is moving the goalposts. I would get a lot more information from them than their gender, race, claim, and name if we do that.