A CERTAIN mellowness suddenly touches relations between the Soviet Union and the People's Republic of China, the erstwhile allies whose falling out 25 years ago transformed international politics as nothing else since World War II. No, the two communist powers are not back in political harness, a development far from possibility and one that would bring a fresh upset in the global balance of power. But with the passing of time and the passing of leaders personally committed to their feud, the fires have been somewhat banked.

The turn came a few years back, after Mao Tse- tung and Leonid Brezhnev. Their successors in effect set a new Sino-Soviet agenda, reducing the priority of the inflammatory issues of ideology, communist movement leadership and disputed territory and taking up a more modest list centering on immediate security tensions. In today's climate, even those security issues appear less urgent; at the least an effort is being made to isolate them from other questions.

Both sides now find it possible to seek areas of practical cooperation. Kinder words and cultural exchanges began some time ago, and the other day three technical agreements were signed -- modernizing some of the old Soviet-installed factories in China, and the like. That Moscow sent and Peking warmly received the Kremlin official who most symbolizes the good old days of Sino-Soviet accord in the 1950s indicates the public face they both want to put on their current tie. A limited expansion of trade is the next target.

Americans sometimes feel a slight crawl at the back of the neck when the Soviet Union and China treat each other civilly. The United States doesn't want to seem to be egging the two on in their disputes, but it cannot help appreciating the geopolitical comforts, for this country, of their split. In fact, the American experience with Moscow teaches that cultural and economic connections take you only so far. Beneath the new Sino-Soviet ties persist the rivalry and distrust that limit the two countries' warming.

A degree of political competition is now evident on all sides of the Washington-Moscow-Peking triangle. With Ronald Reagan having conquered his initial reservations about dealing with China, the Kremlin is moving to deny him an uncontested hand in Peking. With Mr. Reagan also making an opening to Moscow, the Chinese, by doing new business there themselves, are taking out a little insurance. The People's Republic, focusing now on internal reform, seems eager to induce Moscow to start competing with the West in providing the wherewithal of China's economic progress. It's lively, and it's peaceful.