Not that I would recommend trying to get away with something like this, but why didn't you just buy the one-way return in your fiance's name since you were already upgraded on the award ticket? You would have risked her getting the upgrade as an FO, but I can't see how your current scenario is going to work in your favor. As another person said, you can't board yourself on the flight twice, one will be considered a no-show then the seat is subject to being given away to someone else. Scanning you on board secures your seat, not OLCI, which seems to be the impression some folks are under? Unless the OP is under the impression that a GA will scan 2 BPs for him...which is another set of problems altogether.

Am I missing something?

And I don't see how a bag can have a gender? Surely a FA has seen a male carry on a girlie-looking bag before.