As long as the GA scans your two boarding passes, you should be good to go.

Obtaining boarding passes is easy, no one will question anything if you use Mr. Chicken. There's a chance however that the GA may start asking questions, so if I were you I'd just board with the bulk of the crowd--I woulnd't camp on the red carpet

Someone pointed out that musicians can buy two seats... it's true, anyone can do so in theory, but then both seats are on the same PNR.

There's also anecdotical evidence that United's reservation sytem cancels duplicate reservations, but while I've found this to be true for held itineraries I've never seen it happen with confirmed itineraries. Besides, nothing in the CoC prohibits duplicate bookings.

Bottom line: if I were you I'd try, and hope that the GA doesn't start asking useless questions.