Do I need to read Li Zhongguang's Basics of Music Theory and A Concise Course of Basics of Music Theory at the same time when I study music theory by myself? Which is more suitable to choose first?

You don't need to read it at the same time. Just choose one of the two books. There is no question of which one to read first. The general content of these two books is the same, but there are some differences in arrangement and organization. The former actually uses a little more people than the latter.

The most important thing is not which book to read, but whether you can learn all the knowledge after reading it. I suggest you choose either of these two books, and you can ask me any questions during reading.