Two returning champions and one runner-up advanced in Saturday's opening round of the Advocare Cadet Nationals in Fargo, N.D., marking the first of eight consecutive days of wrestling in the FargoDome.     Grant Miller of Roscoe, Ill. began defense of his title at 209 pounds in an impressive fashion with two wins by technical fall and a pin in 10 seconds to open the tournament at 3-0. Patrick Barrentine of West Carrolton, Ohio opened the defense of his heavyweight title with 9-0 and 7-1 victories to finish the day at 2-0.     Last year's runner up at 112 pounds, Steve Anceravage of Bloomsburg, Pa., successfully made the transition to 132 pounds by advancing, but not without a blemish. He lost a closely contested battle to Oregon's Sean Santana by a 4-4 referee's decision. He won his two victories by technical fall, 12-0 and 11-0.    Santana finished the day 3-0 with two other victories by a score of 12-7 and a 10-0 technical fall.    In all, 776 wrestlers hit the mats today with the goal of making it to Monday night's finals. Action resumes at 9 a.m. central daylight time with the Cadets and continues at 4 p.m. as the Junior Greco-Roman wrestlers begin competition in the Asics Junior National Championships.    23 mats line the floor of the FargoDome for non-stop wrestling action through next Saturday's freestyle finals. In all, close to 2,500 wrestlers will compete in this week's tournaments, making it the largest combined tournament in the world.