Whether the performance content of the two days is the same or not depends on the specific concert arrangement. Generally speaking, the concert schedule will vary according to the artist's time and repertoire. Therefore, I suggest that you know the schedule of the concert before buying tickets, so as to better arrange your own viewing plan.
In short, if you want to fully enjoy the Xi 'an concert in Faye Wong, you'd better buy a ticket for two days. Of course, if you only plan to watch the performance for one day, you can also buy tickets for one day according to your own needs.