SEOUL, AUG. 12 -- South Korea's rival opposition leaders have agreed to share power, including equal stakes in a future Cabinet, no matter which one of them becomes their party's presidential candidate this fall, Kim Dae Jung and Kim Young Sam said in separate interviews over the past two days.

The competition of the two Kims, a subject of major importance in Korea's drive toward direct presidential elections, was described by both contenders as under control due to a regular series of meetings they have held in the past six weeks.

Both Kim Dae Jung and Kim Young Sam said their agreement to share ministerial posts between their separate factions dates back to last December, well before the tumultuous events of June ended in government acceptance of their demand for direct elections. In such a contest, which would be the first popular balloting for president since Kim Dae Jung narrowly lost in 1971, the opposition Reunification Democratic Party has a serious chance of winning, according to Seoul newspapers and political observers.

But Kim Dae Jung, in an interview yesterday, said there are reports that some military leaders, and perhaps President Chun Doo Hwan, are not reconciled to the drive toward direct elections, which Chun steadfastly opposed before conceding to opposition pressure June 29.

"The one thing that is unclear is Chun's attitude," said Kim Dae Jung. "We are watching his attitude with concern."

A senior aide to Chun said in a separate interview today that the president supports the June 29 reforms announced by Roh Tae Woo, presidential candidate of the ruling Democratic Justice Party. But the aide acknowledged there had been "no prior discussion" about the measures between Chun and Roh, who is one of the president's closest associates.

The aide said one reason Roh did not take his ideas to the president before making them public was that Roh believed "there were some elements in the ruling party and government" that opposed his two most controversial proposals, direct election of the president and restoration

of political rights to Kim Dae Jung following years of imprisonment and house arrest.

The aide, who asked that he not be quoted by name, seemed to signal a campaign that will include some harsh charges against the opposition. He criticized Kim Dae Jung and Kim Young Sam for statements advocating the release of political prisoners, including some accused of being communists, and charged that the opposition includes "radical students, leftist leaders and even extremist religious leaders" as well as mainstream politicians.

The two Kims reiterated in separate interviews that only one of them will be a candidate in the fall. Many opponents of the government fear that if both opposition leaders run, or if one candidate emerges only after a nasty fight, voters will turn to Roh.

"Not only will we maintain our united front to fight for democracy, but we will continue our united front after achieving our goal," Kim Young Sam said today. "Once before our party was split into two, and we should learn from such a disaster."

Despite their common front at the moment, both men have long aimed for the presidency and neither shows any sign of conceding to the other. In addition, they disagree on significant questions of timing and strategy.

Kim Young Sam said today that the chief priority should be to decide on an opposition candidate quickly.

"The public's daily focus is to see the nomination of one candidate as soon as possible," he said. "The longer we are waiting, the more the public has doubts."

But Kim Dae Jung espoused a contrary view, saying the two should wait as long as possible before designating a candidate. He said a single candidate would give the government a target on which to focus its attacks.

Already the attack on Kim Dae Jung, the more militant and controversial of the two opposition leaders, has become intense. A book attacking him by a former bodyguard has become a bestseller. Independent journalists said there are reports that the book has become required reading in some Army units, with soldiers required to submit reports on what was learned from the volume, which Kim charged was the product of the Korean CIA.

Kim Dae Jung, who has not campaigned extensively through the country since 1971, has postponed to early September a tour of the countryside that is expected to test his popularity.

Perhaps the most important and symbolic stop is the city of Kwangju in Kim Dae Jung's home province, where between 200 and 800 persons were killed in 1980 when military forces loyal to Chun suppressed a revolt against his rule.

It is widely expected that Kim would draw a massive crowd, and anything less might be a political embarrassment. Kim also faces difficult decisions about how militant or how conciliatory he should be in this appearance.

The Kwangju massacre, as the violent suppression is known, has been blamed in part on the United States because of the belief among many Koreans that Washington facilitated it by releasing Korean troops from front-line duty to go to Kwangju.

Washington, while maintaining that there is no truth to the allegations of culpability, has said little for fear of offending the Chun government. U.S. Ambassador James Lilley plans a visit to Kwangju this weekend where he is likely to be questioned about the U.S. role.