SEOUL, South Korea (AP) _ The nation's two leading dissidents joined forces Friday as co-chairmen of the organization that has played a major role in the resurgence of the political opposition in South Korea.

Kim Dae-jung told reporters he and Kim Young-sam would serve as co-chairmen of the Council for Promotion of Democracy.The move had been expected since Kim Dae-jung returned Feb. 8 after two years of self-exile in the United States.

The two opposition leaders met reporters at the home of Kim Sang-hyun, who had been acting co-chairman of the council. They then went into a meeting to discuss the proposed merger of their New Korea Democratic Party with other political organizations.

Kim Young-sam told reporters later, ''Chairman Kim Dae-jung and myself have confirmed that we will do our utmost for the nation, to meet the people's aspirations and do away with any selfish ambitions in the future.''

A statement issued on their behalf said they ''agreed to combine efforts and take a common stand for furthering democratization and the development of the opposition.''

Kim Young-sam formed the council last May, and it was instrumental in establishing the New Korea Democratic Party shortly before the National Assembly election last month.

The party, which also inluded many supporters of Kim Dae-jung, showed surprising strength at the Feb. 12 elections. It captured 67 seats and later attracted the allegiance of two independents.

President Chun Doo-hwan remains firmly in control, however, with his Democratic Justice Party holding 148 of the 276 assembly seats. But the opposition showing led him to reshuffle both his Cabinet and the top ranks of his party.

The two Kims did not actively participate in the election because they were barred from politics. Chun lifted the ban March 6.

Kim Dae-jung was convicted of sedition in 1980 and sentenced to death, but Chun commuted the sentence to life imprisonment in January 1981. That was reduced to 20 years in 1983, and he was allowed to go to the United States for medical treatment.

The conviction also stripped Kim of his civil rights, and he still cannot vote, run for public office or hold office in a political party.The opposition has demanded the government restore Kim's rights, and this is expected to be a heated issue when the new National Assembly meets in early April.