While I find AC's IT is generally abysmal, at least they are upfront about this scenario that you can't have two people nominate an upgrade on the same PNR:

From http://www.aircanada.com/en/aeroplan...des/howto.html

Travel using another member's eUpgrade Credits
Currently, only one member may sponsor an upgrade per reservation. For instance, if a Super Elite 100K and Prestige 25K member are travelling together on the same booking, it is not possible for the Super Elite 100K member to sponsor an upgrade for the departing flight(s), and for the Prestige 25K member to sponsor an upgrade for the return flight(s).