Companies merge every day. Maybe it's time for countries to do the same, voluntarily. For the sake of discussion, let's say the two countries are the United States and China.
Obviously there are too many obstacles, all psychological, to ever allow this to happen. But it makes me wonder what the benefits could be if it happened.
You could start the discussion by imagining that the U.S. and China would maintain their own leaders and laws much the way a state has a governor and its own local ordinances. The new unified Super Government would only deal with the big issues of global security, trade, and accelerating the benefits of leveraging the resources of both countries.
The Super Government would probably need to be made of equal members from the U.S. and China, and require a 75% majority for any decisions. That limits any actions to things clearly benefitting both groups.
The first obvious benefit to this arrangement is that you wouldn't point nukes at your own nation. Second, international trade negotiations would be easier. Few countries could afford to piss off both the U.S. and China. And I am assuming there could be substantial benefits to closer economic and environmental cooperation.
You could argue that the U.S. and China can already get those benefits by agreeing to any actions that are in their mutual interest. But there is something about being labeled the same country that makes agreement more likely. For example, I know that some states in the U.S. get a bigger piece of the federal spending pie, but I'm not bothered because somehow it's all in the family.
Maybe a U.S. and China merger allows for an elegant solution to the Taiwan situation. Toss Taiwan into the merger, giving them one or two representatives in the Super Government, and a veto over any decision directly affecting their people. On one hand it's effectively no change at all, while on the other hand the leaders of China could say they unified Taiwan with China. Ta-da!
You can find lots of reasons why a merger among very different nations wouldn't work. That's no challenge. The fun part is that this thought experiment demonstrates how much we sacrifice to the limitations of human psychology. When you define some other group as part of your own, everything changes while nothing changes.