When to book tickets and seats/berths for two people travelling together

Hello,
my colleague and myself will travel around China in June and need to book tickets for Shanghai-Hangzhou, Hangzhou-Ningbo, and for Z92 from Shanghai to Xian, soft sleeper or luxury soft sleeper. We want to sit together during the first two trips, and we want two upper berths in the same compartment for Shanghai-Xi'An (in case we decide for the soft sleeper). How do we book to make this possible and how much time before the travel can we book at the earliest? Can the tickets be delivered to our hotels (in Shanghai and Ningbo)?
Liliou from France