I did this exact thing just recently. My wife and I were on different PNR's and different seats. I called to link the PNR's and get us seats together and they initially refused, then said, oh you're an elite member, no problem, we can do that for you. I must have checked the AC website a hundred times to ensure that our seats were together.

Of course, when I got to the airport they told me that not only were our PNR's not linked, but that my wife had no reservation at all. After much angst and conferring with supervisors they found my wife's reservation, but claimed the PNR's were not linked and that we did not have seats together, and no, they could not put us together.