In general it's the marketing carrier for the first segment, so UA in your case... but they don't have to include anything that would be included as extra for status if you were flying on that marketing carrier's metal (the *A extra bag). Ultimately it's going to be up to the check in agent. I've had it go both ways, though the time they tried to deny it it didn't matter. The way they were talking, a supervisor could have overridden the computer, but since I had a companion with me who was only using 1 bag from her allowance we just shifted my extra to hers.