Want to visit Hong Kong but have Chinese Visa with 1 entrance only

I get Chinese Visa for 30 days in Chilean Chinese Embrassy (Chile), but they only give me one entrance instead two oír moré because I dont have date to Hong Kong trip. Can I apply to go HK using 72hr free visa, so I can travel closely at end Of my trip in China? Or can I apply for a second entrance to China? Im departing From Guangzhou to New York at Jan 18 non stop. Thank you