GRAIN EXPORTS to the Soviet Union, and automoble imports from Japan, belong to the same dangerous category of double-edged issues. One edge cuts abroad, the other here at home. These presidential choices will affect American diplomacy and national security overseas and, simultaneously, domestic economic policy--not to mention politics. The Reagan administration has not yet had time to work out a reliable approach to the double-edged issues. Left to the traditional scramble among the Cabinet departments, they are proving divisive within the administration, which, in these two cases, is teetering on the brink of a couple of serious mistakes.

Worse luck for the White House, the parallels between cars and corn are awkwardly close. Both industries are going through a rapid and painful process of consolidation that is going to squeeze out or merge in the weaker producers. Both cite campaign promises by President Reagan, who takes them more seriously than some of his predecessors have done. One case touches a crucial interest of this country's chief adversary, the other the prosperity of its principal Pacific ally and the largest overseas customer for American exports.

Regarding cars, the administration doesn't want to impose formal quotas but keeps nudging the Japanese to do something. Commerce Secretary Baldrige seems to be looking around for a deal in which a Japanese promise of export restraint might be matched by promises of wage and price restraint from the American auto makers. That would be a truly sweet accommodation if it worked. But it won't. The Japanese government does not have full control over its automobile industry, and at the first hint of violation, any agreement based on mere winks and nods will come unraveled.

In trade policy, the administration's prevailing interest is to reduce inflation in the United States. Quotas on the Japanese competition mean higher prices for American cars, which in turn mean severe damage to the president's attempts to stablilize the American economy. Does the Commerce Department understand that key point as well as, say, the Treasury or the Council of Economic Advisers?

American agriculture is now among the highest of the high technologies and the rest of the world looks on the American farmer with the same mixture of awe and apprehension that American auto makers reserve for the Japanese. American farmers have always been exporters. That's why the American grain embargo--or, more accurately, the partial embargo--of the Soviet Union has become almost an obsession with the farming interests in this country. But if the United States lifts the embargo totally now, on the transparent grounds that the Soviets have been decent and generous enough not to invade any more of their neighbors, it will leave the unfortunate impression that the American government can't stand prolonged pressure from domestic producers. It would also suggest that the United States was prepared to forget about Afghanistan.

There's a better approach. Some years ago, to hold down the market disruption, the United States signed an agreement with the Soviets to ship a mandatory minimum of grain every year. There were to be additional shipments only if both governments agreed, and it's only those additional shipments that have been affected by the embargo. But the agreement, and the mandatory minimum, expire later this year. Perhaps one possibility for the American negotiators is to split the sales, continuing to tie the present semi-embargo to the occupation of Afghanistan while tying continuation of the minimum shipments to Poland and other matters of concern here.

Beyond the details of cars and corn, these cases demonstratge a need for a better mechanism at the White House, or close to it, to handle the double-edged questions. They have been more frequent, and they have cut more deeply, over the past decade with the enormouse expansion of American trade. Each of the last several presidents has tried one solution or another, but none has been very satisfactory. Worst of all is the habit of leaving them to the traditional structure that assumes a division of labor between foreign and domestic affairs. For a lengthening list of the policies upon which American prosperity and security now turn, that distinction no longer exists.