Requirement for Double Entry Visa

I'm planing visit China for 12 days official event.I have received the invitation for that 12 days event.However,I wish to visit Russia for 7 days as a tourist just after finishing my 12 days event and come back to china.From China I will take my flight back to Colombo.
My question is ,I was invited for 12 days program.Since I planed to visit Russia I have to take Double entry visa.Therefore,what are the requirements I have to fulfill in order to get Double Entry visa ?