We advise reviewers within the same stage that they must communicate and decide amongst themselves who will be the Decision-maker, as it will then lock the stage and lock the other reviewer out when they do.
The problem is, they have no way of knowing if their stage is set to "Only one decision required" or not. So they have no idea if they need to have that conversation or not.
The goal is that the onus will be on the reviewer/approvers to decide who will make the Decision for their department.