Two entries - but only one entry in visa

From September 3rd I'm going to study a year in China, therefore I applied for X1. However, I have decided to travel around China before starting studying, so first we are going to Mainland China and afterwards to Hong Kong. Here I assume I have used my one entry. When my friends fly back to Denmark from Hong Kong, I am going to stay there for a week more or so, and then I will come back to China. But what shall I do? I only have one entry, and it is not possible to apply for a travel entry permit, because you are only allowed to have one visa. What do I do?