Delegates from the two Chinas showed up an international conference yesterday marking the first time in three decades that representatives of Peking and Taiwan have attended the same gathering.

Veteran diplomatic analysts said the appearance by physicists from both China and Taiwan at a science conference in Tokyo signaled a major change in the longstanding conflict between the two governments.

In the past, one side has always boycotted any social gathering, conference or sport meet attended by the other.

"This is the clearest indication we've had that the Chinese are willing to be more flexible on Taiwan," said one analyst. "It's equally interesting from Taiwan's standpoint, suggesting they might be more pragmatic now about demanding that all these groups choose between them and Peking."

Although yesterday's appearance probably represents a change of tactics rather than an end to hostilities in the 50-year-old Chinese civil war, it gives some hope to American policy-makers. U.S. diplomats have had trouble winning Chinese approval for a formula that will bring Washington full diplomatic relations with Peking without forcing the United States to end all security commitments to Taiwan.

The Tokyo meeting "is another sign of China's whole new flexibility in foreign policy," said one diplomat, noting also Peking's new treaty with Japan and Communist Party Chairman Hua Kuo-feng's current trip to Eastern Europe, "but I think this carries them further than they have ever been before."

Diplomats said they now expect to see Chinese at other conferences once attended only by Taiwan. This will promote Peking's "united front" campaign, which is designed to weaken resistance on Taiwan by treating residents of the island as part of one large, happy Chinese family.

Although organizers of the 19th international conference on high energy physics had said both sides planned to attend the Tokyo seminars, the Taiwan delegates were late in registering. It was not clear that both sides would be there until Yang Ying-chuan of Taiwan's Tsinghua University walked into the meeting room at the Keio Plaza Hotel where three Chinese physicists sat waiting for a lecture by two Princeton University researchers.

A crowd of about 200 delegates had already taken all the seats, so Yang leaned against a wall only two chairs away from the Chinese delegation. He moved later when a photographer tried to take a picture including him and the three Chinese.

When asked why his group had agreed to attend a conference that included delegates from Taiwan, Tzu Hung-yuan of Peking's institute of High Energy Physics said: "Taiwan is a province of China. As fellow country-men we are pleased to attend the same conference." Speaking in English, he repeated that explanation three times.

Yang was asked why his government had broken its longstanding boycott policy."I came for a study conference. I'm interested in the study of science," he said, speaking in Japanese. "I don't know about these other matters."

Tzu pointed out that the 800 to 900 conference delegates were listed as representing only their universities or institutions and were not identified by nationality. Yang, when asked, said he received no special instructions before he left Taiwan, Japanese, European and American diplomats all said, however, that the presence of scientists from both China and Taiwan had to be the result of high-level policy decisions by the two governments. Taiwan's government information serv-office had no comment yesterday on the meeting.

Attending the conference may represent even more of a departure for Taiwan than for China, for Peking in recent months has given several signs of a new approach to dealing with its Nationalist Chinese adversaries Rep. Lester Wolff (D-N.Y.) said last month after visiting Peking that the Chinese had expressed a new willingness to negotiate directly with the Nationalists about their differences. The offer seemed mostly an effor to win sympathy for Peking in the United States, since Taiwan was unlikely to ever consent to such talks.

This month in a joint communique announcing full diplomatic relations with Libya, one of the few remaining nations with an embassy in Taiwan, Peking bowed to Libyan wishes and kept all references to Taiwan out of the joint communique. As far as is known the Libyans have yet to close their embassy in Taiwan or force the Taiwanese diplomats to leave Tripoli. A Chinese editorial on the Libyan agreement did, however, contain the usual denunciation of Taiwan.

Peking and Taipei continue to engage in a fierce propaganda war, but but there has been no serious military clash between the two in 20 years. China shells the Nationalist-held islands just of the coast of Fukien Province with propaganda leaflets. China apparently has neither the desire nor the capability to invade Taiwan at present but Taipei and Washington are worried about Peking's future plans.

China's presence at yesterday's Tokyo conference indicates its deep interest in reviving scientific research and establishing contact with foreign scientists after several years during which Chinese political struggles left little time for natural science.

"In the past, when Taiwan was stronger diplomatically, they probably would have boycotted the conference if the Chinese had showed up," said one longtime analyst of Chinese foreign policy. Since the beginning of the 1970s Peking has suceeded, however, in establishing relations with dozens of former friends of Taiwan and "Taiwan's diplomatic position is now so weak that they apparently think it is more important to maintain whatever international contacts they have even if the Chinese are there."

Conference officials said five Chinese delegates registered at the conference. The Peking Institute's Tzu was joined by two other Chinese physicists. Tai Yuanpen and Huang Tao, at the morning lecture on "confinements" conducted by C. G. Calan and D.J. Gross of Princeton.

Two other physicists from Taiwan have reportedly registered along with Yang. The Chinese have rooms on the 21st floor of the ultramodern hotel in Shinjuku. The Taiwan delegates are on the 18th floor. There has so far been no reported contact between them.

Taiwan President Chiang Ching-kuo has denounced any suggestion of talks with Peking, saying "negotiation with the Communists is tantamount to suicide." Diplomats expect that if any contacts have or do occur they will be kept very secret to avoid a business panic on Taiwan.