The two Salvadoran men who bumped into each other in a Los Angeles restaurant last night had not met in years, but what years they had been. Oscar Acevedo, plump and amiable, is no longer a university professor of philosophy: he went through a career switch in the early '80s and joined the guerrillas of the Farabundo Marti Liberation Front, for which he is now a spokesman.

His former colleague in the humanities department of the National University, the courtly, soft-spoken Juan Ricardo Ramirez Rauda, spent the past few years living abroad working with an international finance organization. He had been a Christian Democratic Party leader when the party and its candidate, Jose Napoleon Duarte, were deprived of an electoral victory by an Army coup in 1972.

Now, Ramirez told Acevedo, he was awaiting confirmation of his appointment as ambassador to West Germany.

Their brief accidental meeting in the restaurant of the hotel, where both were staying, was charged with more emotion than any of their exchanges in the tense but civil day-long televised debate today that had brought them to Los Angeles.

In the tiny world of Central American politics there is one overriding fact: that all politics -- whether of the left or right -- is conducted by a minute elite whose members are connected either by work or social relations or family ties.

Despite last month's unprecedented peace talks in the Salvadoran town of La Palma, the parties Acevedo and Ramirez represent are still bitter enemies.

Both the government representatives and the rebels emphasized that the debate was intended to present their views to a U.S. audience and was not part of the "ongoing process of dialogue" that began last month in La Palma.

Nevertheless, both sides clarified their positions in the course of the debate, which was sponsored by the Center for the Study of Democratic Institutions and moderated by its president, historian Allen Weinstein.

Speaking for the rebel Farabundo Marti front, Acevedo said that the guerrilla coalition and its ally, the Democratic Revolutionary Front, would be willing to accept a bilateral commission to monitor an end to the flow of arms into the country from any outside source.

"This would be an important first step which would really lead to a political solution and toward a cease-fire as quickly as possible," Acevedo said.

The head of the ruling Christian Democratic Party, Julio Adolfo Rey Prendes, confirmed that a second round of talks would take place later this month. The guerrillas, through official intermediary Archbishop Arturo Rivera y Damas, have proposed Nov. 27 as a date and San Salvador as the location, but Rey Prendes said he had not received these proposals. He appeared to rule out any cease-fire soon, saying that his government had called only for a "cessation of violence," not a formal cease-fire.

Although it had been announced that President Duarte was planning to participate via remote hookup from San Salvador, he decided against it yesterday, according to his spokesman, Gerardo Le Chevalier. The rebel coalition responded by withdrawing the participation of its president, Guillermo Ungo.

Another scheduled participant, Democratic Revolutionary Front spokesman Ruben Zamora, was not allowed a visa.

Yet two members of the military wing of the rebel superstructure were given visas, the first known to be issued to that militantly Marxist grouping.

The far right was represented in today's debate by Francisco Quinones, the leader of the Salvadoran Popular Party, an articulate supporter of the Reagan administration's Central American policies.

In addition to Rey Prendes, the government was represented this morning by Economy Minister Ricardo Gonzalez Camacho and by ambassador-designate Ramirez. In the afternoon session, the government offended the rebels by sending in their second team -- three lower-ranking Christian Democratic officials -- to continue the debate.

But for high drama, nothing today compared with the moment the previous evening when three reporters sitting at dinner in the hotel restaurant watched Ramirez and another official, presidential spokesman Le Chevalier, walk into the room. At the table with Acevedo were two other rebel representatives: Hector Oqueli, the sophisticated Social Democratic member of the rebel political alliance, the Revolutionary Democratic Front, and Salvador Samayoa, a former minister of education who had dramatically announced before an auditorium full of university students in 1980 that he was leaving the government to join the most radical of the five guerrilla groups that make up the Farabundo Marti front.

As the two government officials were led by a waitress to a table immediately next to ours, they stopped, blinked and recovered quickly. There were handshakes all around, and after a brief pause, the officials accepted the rebels' invitation to sit down.

"Aha, you," said Acevedo to Ramirez, in the typical Salvadoran greeting, "how's the family?"

"So how's your twin brother?" Le Chevalier asked his grade school chum Samayoa before ordering poached salmon with capers. Samayoa said his brother was well and was studying in the United States.

The conversation flowed naturally for a while with a series of "whatever happened to?" questions but reached an uncomfortable halt when the chit-chat ran out.

Both sides seemed equally uncertain about the format and purpose of the next day's debate. "Frankly," Acevedo grumbled, "we should at least get together and insist that it be held in Spanish." (In fact, it wound up being held in English.)

Shortly afterward, another government official entered the restaurant and hastily withdrew. A few minutes later, Le Chevalier was paged. He returned, his face was somber. "They said we must leave," he told Ramirez, and the two men quickly paid their bill and walked out.