What two european countries finally united under one ruler as a result of nationalistic feelings?

The answer is Austria and Hungary.

The subsequent country was known as the Austro-Hungarian Empire. It was a powerful state that existed between 1867-1918 when it disintegrated after defeat in World War I.

For more than half a century, the Empire was a powerful state in European and global affairs.

Interestingly it also had two capitals, Vienna and Budapest, both of which became the capitals of Modern Austria and Hungary.