One question on your objections: how would you characterize the state of two human rationalist wannabes who have failed to reach agreement?

I would say that one possibility is that their disagreement is common knowledge, but they don't know how to reach agreement. From what I've learned so far, disagreements between rationalist wannabes can arise from 3 sources:

• different priors
• different computational shortcuts/approximations/errors
• incomplete exchange of information

Even if the two rationalist wannabes agree that in principle they should have the same priors and the same computations, and full exchange of information, as of today they do not have general methods to solve any of these problems, can only try to work out their differences on a case-by-case basis, with high likelihood that they'll have to give up at some point before they reach agreement.

Again this is in striking contrast to actual human behavior even among wannabes.

Your suggestion of what rationalist wannabes should do intuitively makes a lot of sense to me. But perhaps one reason people don't do it is because they don't know that it is what they should do? I don't recall a post here or on OB that argued for this position, for example.