To be frankly, your plan is complicated, in terms of which you have to apply for a double-entry visa of China. I think you would like to arrive at and leave from HK, right? So, why not arrange your itinerary like this: HK-Shenzhen-Shanghai-HK? In this case, you just have to apply for a one-time entry visa to China. On the other hand, you don't need to apply for any HK visa but just hold your valid passport~