ANY AMERICAN president dealing with the Russians must conduct two sets of negotiations simultaneously. This is the reality, sometimes the curse, of diplomacy in a democratic society. The bargaining that must go first, since otherwise there is no foundation for the second, is with his own citizens and allies. The second is with the Kremlin. Yesterday President Reagan went a long way toward completing that vital first set of negotiations. He presented the outlines of the altered proposals on limiting intercontinental missiles that his negotiators have carried back to the START table in Geneva.

"When I established the Scowcroft Commission," Mr. Reagan said, "I could not then foresee the impact that this outstanding panel would have." He was referring to the bipartisan presidential group whose balanced recommendations on arms and arms control, as filtered through an attentive Congress, have now become administration policy. For these recommendations, which have provided Mr. Reagan the politician's dream gift of a second chance, too little credit has been given to Brent Scowcroft and his colleagues. They did a terrific job. You could say, of course, that it is easier to give good advice than to take it. Mr. Reagan has taken it, where a more prideful man might have hesitated to make the implicit confession of earlier error.

The upshot is that Mr. Reagan now has a negotiating position that has been tested and improved in the American political fires. It is an advantage that will not help him so much in the ratifying stage, where he did not stand to need help anyway if he got that far, as in the negotiating stage, where he needs a great deal of help. The support that his new position and, perhaps even more, his newly projected flexibility will bring him should make it harder for the Soviets to go over his head to the American public, as they like to do.

Two broad questions still need to be asked about Mr. Reagan's newly enunciated START position. Would an agreement based on it make the United States more secure? By and large the people knowledgeable about defense believe it would. Is it negotiable? That is, can it be matched to the Soviet Union's own definition of its self-interest? We think it can be. Our distinct impression is that Mr. Reagan is becoming, by political necessity if not by personal choice, a believer in arms control as one essential element in strengthening American security. Gone from his latest statement is his frequent past intimation that arms control was unnecessary, perhaps even a peril. His appeal to the Soviets can be usefully underlined: "To the leaders of the Soviet Union, I urge that this new opportunity not be lost."