Us citizen to Beijing with excursion to Cambodia then to Beijing for flight home.

We are planning a trip to Beijing for vacation in April. We will be in Beijing for approx 4 days then plan to fly to Cambodia for approx 7-8 days then return to Beijing for the flight home. I understand we need a double entry China visa but will they approve it for the excursion to Cambodia? How soon should we request the entry permit? We want to purchase our tickets and make reservations soon.
Thank you.