For a reported $2.1 million apiece, World Boxing Association light heavyweight champion Michael Spinks and World Boxing Council light heavyweight champion Dwight Braxton will meet in a 15-round title bout tonight in Atlantic City that should produce only the second champion to be recognized by both associations.

The winner of the bout, which is being hyped as "the brawl for it all," will join middleweight champion Marvin Hagler as the only joint WBC and WBA champion. The fight will be shown on closed-circuit television at Capital Centre at approximately 10:15, after the Washington Bullets-New York Knicks game.

Oddsmakers have Spinks, whose record is 22-0, with 16 knockouts, a 6-to-5 favorite over Braxton, 19-1-1, with 12 knockouts. Spinks, the younger brother of former heavyweight champion Leon Spinks, is considering moving up to the heavyweight division if he wins tonight's fight, in the 14,000-seat Atlantic City Convention Hall.

Spinks, who won the WBA championship in a unanimous 15-round decision July 18, 1981, over Eddie Mustafa Muhammad, has defended his title five times since and knocked out his opponent each time.

At 6 feet 2 1/2, he is almost eight inches taller than Braxton, and has a standup style of fighting with a wide variety of punches of knockout potential. His lanky frame makes it difficult to get inside to hit him.

Braxton, who won the WBC championship Dec. 19, 1981, when Matthew Saad Muhammad's handlers stopped the fight in the 10th round, is the shortest light heavyweight champion in boxing history. Described as one of the last of the bob-and-weave fighters, he is considered a brawler who tries to get in close to an opponent and then hammer away at him.

Braxton has defended his WBC title three times, winning each fight by a knockout.

"I'll be so close to him that people will think we're Siamese twins, with my head connected to his chest," Braxton has said. He is predicting a victory by the eighth round.

Spinks, one of five American fighters to win a gold medal at the 1976 Olympics in Montreal, says he's not worried.

"He's vulnerable to everything I throw," United Press International quoted him as saying. "I'll hit him with everything--left jab, left hook, rights, right uppercuts. I have four main punches he's gotta watch out for."

A hard puncher with both hands, Spinks has a reputation as a patient fighter who likes to look an opponent over in the early rounds while waiting for an opportunity to land a damaging punch.

The fighters have known each other since 1980 when Braxton, who served 5 1/2 years of an 11- to 15-year sentence at the Rahway State Prison in New Jersey for armed robbery, was hired by Spinks as a sparring partner.