There's a difference between nested and end-to-end. All the major booking sites, including AA.com, do end-to-end tickets all the time on one PNR.

For example, if you want to fly ORD-BOS on short notice, AA will often offer you a ORD-BWI round trip end-to-end with a BWI-BOS round trip. If you look at the fare codes, that's how it's constructed - you could buy it as two individual tickets if you were so inclined and pay almost the same amount (the taxes might differ a little).

If you want to be "safer" then fly SEA-ORD-LHR as one ticket on AA, and LHR-YVR as the other ticket on BA, and take the bus service or pick up a one-way car rental between YVR and SEA.