Never say never. I traveled with a friend last year to Barbados on American. I was on a heavily discounted fare, my friend was on a 30K mileage award. I confirmed an upgrade for my seat two days prior.

When we checked in, I asked about the upgrade, and ticket counter agent said "do you both want to upgrade?" I said sure, and she did it, using additional upgrade stickers. I know she was bending the rules (I was a Platinum at the time), but First wasn't full and she was in a good mood -- I didn't question it!