Plan to visit Chengdu n Xian 7 days in November? How to go about/ itinerary? by bus/train?

You can either go to visit Chengdu or Xian first. The best way between the two cities is by air which is fast and comfortable.

You can spend 4 days in visiting Chengdu.
Day 1: Arrival Chengdu, begin the tour with a visit to Wangjiang Tower Park.
Day 2: Chengdu - Bifengxia Panda Reserve Base- Chengdu
Day 3: Chengdu - Leshan - Mt. Emei
Day 4: Mt. Emei - Chengdu
Then spend the left 3 days visiting Xian.
Day 5: Arrival Xian, go to visit the Beller Tower, the City Wall,Forest of Stone Steles Museum, Shuyuan Gate, the Big Wild Pogoda
Day 6: Terra Cotta Warriors and Horses, Huaqing Hot Spring
Day 7: Shaanxi History Museum
The best way between the two cities is by air which is fast and comfortable. For details, you can look at this webpage: https://www.travelchinaguide.com/china-flights/