What is this preposterous idea ? History has taught us (in Europe) that two different linguistic groups, and two countries with such a distinct history (not to mention geographic barrier) cannot sucessfully be merged under the same administration.
Look at a country like Belgium. Small as it is (smaller than Kyushu), it has 2 main linguistic regions (Dutch and Frennch speakers), and although the country has been part of the same political entity for over 500 years, share the same religion (Roman Catholic, although nowadays many people aren't religious), there are still strong tensions between the 2 communities. In fact, they are now separated in 2 states (+ Brussels as a bilingual city-state), because otherwise the country would have split up. There are still people who think the country should be divided in two, as the only things that still keep them together is the king (but anti-monarchist would be happy to see him gone), the army (but within the EU and NATO, it's quite irrelevant), and now not even the currency. Every ministry but finance and defense are independent : education, culture & tourism, economy, construction, etc.
It's already hard to imagine how China does to keep itself together. It had to give automony to Guangxi, Tibet, Xinjiang and Inner Mongolia that were too culturally different. I just can't imagine Japan and China together.