With the first meeting of Secretary of State George Shultz and Soviet Foreign Minister Eduard Shevardnadze, in Helsinki, the two superpowers have formally begun planning for a summit that, despite an earlier administration tendency to play it down, increasingly bids to become one of the pivotal events since World War II.

It will enable Ronald Reagan to take his first personal measure of Soviet policy in the period of potentially heightened activism that was suggested by Yuri Andropov and that may yet become concrete under the 54- year-old Mikhail Gorbachev. For the Kremlin it will be the first occasion for an up-close reading of American policy since the conservative wave still ascendant in Washington started flowing in the late 1970s.

It is a mistake for the great powers to go so long without a summit. It deprives their leaders of the opportunity to practice the basic political skill: putting personal assessments of the other fellow into their policy equations. It builds an unnatural tension into summits that do take place. Ready or not, however, November is coming near.

Unfortunately, nothing that was visible at Helsinki alters the previous impression that the two powers are lurching toward a summit in a rather casual way. The summit, with all of its natural freight, is not going to be allowed just to happen. A tremendous extra freight is being piled on.

The Kremlin started it in April by announcing that it would stop deploying new SS20 missiles against Western Europe -- it had been deploying them weekly for eight years -- until November. If by then the United States did not halt deployments of new American missiles in Europe (deployments it began only at the tail end of 1983 to counter the Soviet SS20s), Grbachev said, watch out.

So next November was bound to be a tense East- West time anyway. The scheduling of a summit to coincide with the deadline for the Kremlin's unilateral SS20 moratorium puts the whole European issue squarely on the Reagan-Gorbachev plate.

That's not all. In June Reagan made his own heavy contribution to the November sweepstakes. As part of his "go-the-extra-mile" decision to stick with the terms of the SALT II arms-control agreement, he challenged the Soviets to "correct their noncompliance," and he commissioned the Pentagon to tell him, by Nov. 15, what arms programs the United States should add to offset uncorrected Soviet violations. He promised to submit these programs in the defense budget that goes to Congress early in 1986.

In this context, too, November was already going to be a zinger. Reagan was finally going to carry off his ambitious and politically charged effort to cope with the lagging Soviet record of compliance with old arms-control agreements, even while trying to negotiate new agreements with the Soviets at Geneva. The summit falls precisely as Reagan's Nov. 15 deadline comes due.

That's not all, either. Just this week Gorbachev upped the ante further; upped it, moreover, knowing that he was playing straight into the summit. I refer to his announcement that the Soviet Union has begun a unilateral moratorium on nuclear testing, a pause due to expire at the end of the year unless Washington follows suit.

It was a patent attempt to draw public attention away from the nagging verification issue; it actually cuts across the Geneva arms-control talks, where the ostensibly mutual effort is to negotiate binding restraints. But there it is. Factor in the consideration that a new Soviet five-year economic plan is in the making and that by some calculations the Soviets are today where we Americans were five years ago: ready to go to the next generation of weapons. You will then see that the Kremlin is talking not only about resuming testing but also about picking up the pace of major new weapons programs. That's heavy.

Reagan and Gorbachev: the two leaders are without international high-stakes negotiating experience. They are acting at a moment when Soviet-American relations have not been worse in 30 years and when the great issues lying between them are at best in a state of paralysis. One of these men is recovering from a cancer operation and the other has as his foreign minister a provincial apparatchik who cannot conceivably be ready for the big time.

One might have thought that the two would be edging only warily toward the first summit for either of them and that they would have tried to build some confidence and momentum before taking up the hardest questions. Instead, each, responding perhaps first to domestic imperatives, is turning up the heat under the other and multiplying the stakes of their rst encounter. Each seems confident, without evident reason, of his personal bargaining prowess. Some care and sorting out, and some modesty, are desperately needed.