Travel for several weeks back and forth from Hong Kong to China

Hi,
In may/june I'll be travelling to Hong Kong. There I'll be staying with the parents of my girlfriend, who live there. We're planning on taking multiple short trips to China from Hong Kong, but after each trip I'll be coming back to Hong Kong. So I'll be needing a multiple entry visa. Could this be achieved with two separate hotel-reservations and an extra letter of invitation by a friend who lives in Shanghai ?