I would contend... that if the ORD-YYZ-HKG is fared (based on fare construction) as a through fare, that it should be 1 cert. Somehow I think you're going to be asked for 2 though.
I'm going to be trying something similar in Sept. LGA-YUL-YYZ(18hr connex)-YYZ-LHR and although I believe I should only have to surrender 1 cert, I'm expecting to be asked for 2 certs.