Complex sentence: For the first time the same city, San Diego, hosted both the Super Bowl and the final World Series game in the same year; not only were they held in the same city, they were both held in the same stadium.
Split into simpler sentences: For the first time the same city, San Diego, hosted both the Super Bowl and the final World Series game in the same year. Not only were they held in the same city, they were both held in the same stadium.