You can ask them to *cross-reference.* But that does not have much to do with seating. Might help in case of irregular operations though, such as, rebooking on the same flights. Which might also be a mixed blessing since the optinos for Aeroplan tickets are limited to rebooking on Star alliance.

I believe for Aeroplan rewards you should be able to select a seat. Then for the paid ticket, just pay for seat selection if it's important enough to sit together to justify the cost.