In Aumann's agreement theorem the two rationalists have common knowledge of the subject and share common priors. These two conditions are not true in the common cases you are discussing. Two rationalists can agree that it is not worth while to take the time to make all pertinent knowledge common and come to common priors. This can only happen with unimportant topics because otherwise it would be worth spending more time on it, this is still not agreeing to disagree.

If both rationalists have a high probability of very different answers(strongly disagree) it can indicate, drastically different knowledge or priors, and in real life where no one is perfectly rational it often indicates a lack of rationality in one or both parties. If it is the last case it is probably worth discussing the unimportant topic just to uncover and correct the irrational thought processes in both parties. So if two rationalists that share a large common knowledge base and strongly disagree there is a higher probability the disagreement arises from irrationality on one or both parts and it is therefore a good idea to discuss the topic further to isolate the irrationality and correct it.

On the other hand if the two rationalist have very different knowledge bases then it is likely their disagreement arises from their different knowledge bases and/or priors. Sharing the two knowledge bases could take a great deal of time and may not be worth the effort for an unimportant problem. If the two rationalists decide to walk away from the with out sharing knowledge they should both devalue their knowledge proportionally to how much they judge the other to be rational(both in logic and ability to curate their data and formulate priors and taking into account that it is harder to judge the others irrationality dues to the large difference in knowledge bases).