PRESIDENT REAGAN on Tuesday oversaw the signing of an agreement with the Soviet Union to set up ''nuclear risk reduction centers'' in the two capitals. The event reflected and warms the atmosphere in which the two superpowers are currently working toward more conspicuous agreements. But the new agreement, which results from unusual lobbying in Moscow as well as Washington by Sens. Sam Nunn and John Warner, has its own importance in raising the profile of an area of overwhelming common concern.

The agreement commits each country to open a nuclear risk reduction center in its capital to keep 24-hour watch on ''events with the potential to lead to nuclear incidents.'' It's more complicated than it may seem. Nuclear risks come in two categories. Those that are commonly though far from universally accepted are the risks that a government creates and tolerates, even as it tries to minimize them, in the course of building and deploying nuclear weapons. A government relying on a strategy of nuclear deterrence will not want risk reduction to inhibit the organization or conduct of its defense, and the agreement does not invade this sphere. The other kind of risks arises from ''accident, miscalculation or misunderstanding,'' in the language of the new agreement, whose operating premise is that an adequately sharp line can be drawn between the two kinds.

Identifying risk reduction as a separate government function, one to be performed by its own office or bureaucracy, is new and raises considerations of turf, management and efficiency. Ideally, after all, the whole executive branch ought to be a nuclear risk reduction center, and there should be no need for any responsible official to be urged to tend to this supreme task. The practical difficulties -- of sharing information and intelligence, for instance, or of communicating in a crisis -- have induced the two governments to go slow. They are holding off on joint manning of the centers. They have given no specific mandate to the centers, but evidently are prepared to test certain possibilities of cooperation on nuclear terrorism or on nuclear threats by third parties. The initial emphasis is to be on rerouting certain routine, already existing exchanges of nuclear information through the new facilities. The shared intent seems to be to explore what usefully can be done. At least it's a start.