Train tickets for Chengdu to Beijing via Xian

I am only stopping off in Xian for one day on my travel from Chengdu to Beijing.
Should I make the bookings for 2 separate trips (Chengdu-Xian and Xian-Beijing) or for one whole trip Chengdu to Beijing?
Will the train tickets for the trips be delivered to my hotel in Chengdu as I am not staying in a hotel in Xian?