i think that because both do not start and end at the same city, not because of stopover. I think if you try YYZ-YVR (stop) YVR-HKG (stop) HKG-YYZ (with or without connecting at YVR) will be fine.
Open jaws do price online when I enter them as connecting flights - but if I force a single stopover on the return, it complains about multiple stopovers. It does appear to function as it should if the stopover is on the outbound however.