That was my first thought too but they are happy to take Aeroplan points, an e-coupon and my credit card!
To answer one of my own questions, apparently calling in will not help as Aeroplan agents can't use two either.
There is a restriction noted that only one can be used when making changes to an existing booking, such as seat or travel options, due to system constraints. Not sure how it would be remotely applicable to Aeroplan bookings, but might be the reason why it won't work.