I think you have chosen the right destinations: Beijing and Xian, both are historical and cultural cities in China. I suggest you spend 2-3 days in Beijing to visit the Forbidden City, Temple of Heaven, Badaling Great Wall and Hutongs and 1-2 days in Xian to visit the Terracotta Warriors Museum, Big Wild Goose Pagoda and the provincial history museum. I guess you would get anything you like to see in these two cities.