China 6 days then Hong Kong 5 days and back via Beijing airport

I'm flying to China, spending 6 days in Beijing and then flying to Hong Kong for 5 days. i will fly back to the UK via Beijing airport (I think we have about 2 hours wait in the airport). We are staying airside at beijing airport on the way back from Hong Kong. Would we need a double entry visa, or would single entry be ok?