A double entry visa is generally valid for 6 months, with 30 days duration each entry. You need to provide the probable date for each travel to the application form. And visa officers will take it into consideration. Multiple entry visa is possible that you can write this request to application form either, As I know, if you want a multiple entry visa, you need to provide proof that you have visited China twice in last year, and will go there frequently this coming year. So, you'd better contact a travel agency appointed by Busan Chinese consulate if you need more documents.