Does one of you have status, and the other doesn't?
I'm not sure the extent to which this still exists, but in addition to "preferred" seats, IIRC there are still what is referred to as "priority" seats on AC. "Priority" seats are non-preferred but near the front of the airplane and are a bit of a throwback to the pre-"preferred" days. The Altitude website still references this. Priority seats are generally not shown as being available to people who aren't eligible to select.
https://altitude.aircanada.com/statu...ram-privileges
Under "Priority Seat Selection"
Take a seat at the front of the Economy Class cabin on flights operated by Air Canada, Air Canada Express and Air Canada Rouge. Seat selection fees may apply with some fares.
The only difference in seat maps is Rows 4-6 which leads me to believe 1 pax is eligible for priority seating and the other is not. If one of you is eligible, you might have luck calling AC reservations to get a seat assignment together, but YMMV. Also the fact that this is a UA codeshare could complicate seat selection.