Not sure which two carriers you want to have these flights credited to, but just a warning about that YVR-JFK-YVR trip if it is booked in M calss: it will qualify for mileage on Cdn+, but is an ineligible fare for AAdvantage. And you will also have to argue with CX agents to even get them to enter you AA number onto the file since it is not a mileage accrual fare. AA has also adjusted the fine print on the 100K oneworld promotion (I assume you want to use this for a 4th or 5th partner on each carrier) to clarify that non-mileage accrual flights are not eligible. This was not the case in their original material.

If you are elite on one of the airlines you want miles credited to, use that as the first carrier on the outbound flight, and let the number stay in the file. Most flights out of JFK are heavily booked and upgrading of oneworld elites is common. That was my experience last month ex-JFK. Just don't forget to change your FF program number on check-in.

Also, always use the J or F class check-ins. These are staffed by more senior agents who are the ones that can make changes in the computer as a matter of course. Newer agents often don't have the same level of familiarity and find this rather more challenging.