Knowledge ascriptions are factive, but many of us are fallibilists about knowledge. So, I hope that this isn't an argument that would move many people to think that there can be false, justified beliefs:
(P1) It is possible to justifiably believe p even if you have the same justification(s) for believing p that someone does in some ~p-world.
(C) It is possible to justifiably believe p even if ~p.
I think this isn't a good argument [why not?], but I do wonder if it is this sort of argument that helps explain why all of the orthodox accounts of justification allow for the possibility of false, justified belief.
There's a reaction to this sort of argument that is just as bad as the argument. There are passages in McDowell where it seems he urges us to flip this argument on its head. He doesn't use the language of justification, but there's the thought that knowledge is a standing in the space of reasons and the internalist thought that epistemic standing cannot be beyond your ken or blankly external to you. This might add up to a sort of infallibilist view on which (C) is rejected which is taken as reason to reject (P1).
I try to untangle the web a bit in the latest draft of my disjunctivism paper (here).
I've also posted a revised draft of my paper, "Belief's Aim and its Justification". In it, I argue that if reasons for action are facts, justification ascriptions are factive. See here.
Why is it not a good argument? Whether something is justified depends, in part, upon the reasons to/for and the reasons against. Even if two subjects have the same reasons for V-ing, V-ing might be justified only for one of them because they might differ in terms of the cases against their V-ing in the situations they face. So, the argument assumes that there will not be differences in the reasons not to believe when we move from a situation in which a belief is true to a situation in which it is false. Why would anyone think that? Is there no reason not to believe the false?