They will be able to agree what should be uncontroversial, and about many forms of criticism. They will agree on what facts are reasonable to believe, even if they choose differently. When there is a continuum of positions on a subject, even if they do not agree about quite what the right spot is, they will be able to understand why the other is further in whatever direction, and agree that each is being reasonable, even if perhaps mistaken. Why reasonable, if wrong? Because they will know that their arguments for the specific place on the continuum, are not so uncontroversial and precise as to necessitate reasonable people to agree.
My current view is that the worst type to person to try and talk to about serious stuff, is not the one with some bad moral theories, but rather the one with bad epistemic theories. (Note that a certain minimum morality is required to hold a good epistemology, so moral inverters are not gonna pass my epistemic criterion. Mainly what's required for good epistemology, is valuing truth-seeking, or something along those lines. And note that valuing means people without values are out.)