To celebrate the season finale of The Voice, Christina Aguilera and Lady Gaga performed together for the very first time on Tuesday, Dec. 17, 2013 in Universal City, Calif. They never even got a chance to practice it together. On stage was the first time seeing each other. The long-rumored rivals showed no signs of animosity as they belted out the latter's hit, "Do What U Want," in matching lamé outfits.