Can I visit the city wall and the terracotta army in one day?

Generally, your time can be ok to cover both of them. After arriving, take a coach to Lintong in an hour, then catch a taxi to the Terracotta Army for a visit. The visit may require 3-4 hours if you are fairly interested. Afterwards, take Tourist Bus Line 5 (or no. 914 or 915) back to Xian Railway Station in an hour. Then take a taxi to the city wall, the south gate of which is open till 22:00.