LOS ANGELES (TheWrap.com) - "American Idol" and "The Voice" might be rivals in the singing-competition arena, but the two shows will come together - sort of - for the Christmas season. "Voice" coach Cee Lo Green and Mariah Carey - who begins her stint as an "American Idol" judge when the show returns in January - will both be on hand to perform on NBC's 15th Annual "Christmas in Rockefeller Center," the annual celebration of the lighting of the Christmas tree in New York's Rockefeller Center. ...