It might be easier for people of the same nationality to achieve this. A couple which comes from France and Germany, for example, will probably find unity more difficult to achieve than two Germans. If the couple comes from the Orient and the Occident, it is probably even more difficult. They may not realize it in the beginning, but by living together they will gradually discover their differences. If they can achieve harmony before this, the differences won't matter so much.
Unity is easy when they are unaware of their differences.
But genuine harmony and unity can only come after the differences and the resulting problems have been confronted and resolved. I recently read about Europeans who had studied in the Orient. As students it was easy for them to mix with Japanese and Koreans. But when they returned to the Orient after becoming professionals and leaders in their own countries, they experienced difficulties they hadn't encountered the first time. After taking responsibility in their own countries and gaining experience, they found it difficult to unite with Orientals.