Or, since Aeroplan now allows one-way redemptions at half the roundtrip price, you can book 2 one-ways with United miles and two one-ways the other direction with Aeroplan. Amex transfers to Aeroplan are instant so you can transfer as soon as you find available seats. The lowest surcharges on Aeroplan will be on UA or US flights so if you can find a routing using US or UA for the TATL flight use the Aeroplan miles for that direction.

This approach will keep both of you on the same PNR each way.