In a pair-bond comprised of humans, decisions should be made by either a benevolent dictator or by consensus. If the pair is comprised of rational decision makers, then consensus should be utilized. If instead one's decisions are based on irrational "feelings" then a benevolent dictator should have the final word.
If logical, rational, and structured thought form the basis of both parties decision-making process than they should always strive to reach a unanimous decision. Problems can be hammered-out with a logical discussion and the most logical course of action to solve the problem will bubble to the surface. If emotional issues surface and cause a lack of objectivity in one or both of the parties then this will be recognized and weighed into the discussion. Once everything is weighed in, a best course of action will be undeniable and agreed upon. If it does not work out that one of the party concedes to the correct course of action, then chances are that they are acting irrationally and not following a logical argument. Remember, even though it might turn out that it is wrong in the future, there is still a "best course of action" that can be decided upon given the current data.
If two people cannot base their decision making process on sound logic, then it is better to designate one as the benevolent dictator. This should be the person that is shown to make better decisions more often. If you are wrong more often then not, what makes you think you are correct about this one particular issue? You are better off letting someone that is correct more of the time make the decisions. When two people argue without sound logic, then the outcome is hopeless. Both parties are providing unsubstantiated arguments which neither has any reason to believe over their own unsubstantiated and illogical reasoning. Both think they are correct and since their arguments are not based on logic, they cannot be disproved by logical means. Nothing will be resolved and only hate and anger will come from such a futile activity. Because of this, only a dictator that has the interests of both parties in mind, should be allowed to make the final decisions. It has to be trusted that the chosen dictator has the couple's best interest in mind.
If neither of these conditions can be met i.e. a logical partner or a benevolent dictator, then joint decision making should not be undertaken. The people should remain as individuals and not pair up.