How do you put two conductors, two pianists, a composer and a cellist on the stage at the same time with the smallest possible number of people? You arrange twin billing for Mstislav Rostropovich and Leonard Bernstein.

It was fascinating to watch these two musical giants rehearsing the National Symphony Orchestra last week. Double rehearsals were routinely followed by periods of overtime as Bernstein, the composer-conductor, honed his new "Songfest" to the artistic precision he wanted for its world premiere, while Rostropovich, new at conducting Bernstein's music, polished the suite "On the Waterfront" until he had it sounding the way the composer said later he wished it sounded on his own recording.

Then there were those times when the composer-conductor held the baton and the conductor-cellist held his cello and bow in readiness to play the first performances of Three Meditations from Bernstein's "Mass." In an instant the relationship shifted. The composer asked the cellist to hold the B a moment longer. "Let it sound a fraction more before you leave it." And later, again composer to cellist, "You need to make a larger sound here because the orchestra had just been making so much noise that we don't hear you when you first come in."

The two men were discussing their interrelationships at a post-rehearsal press conference. Bernstein was asked how he felt about working with Rostropovich. "But we are colleagues," he said instantly. "It doesn't matter whether we are playing the piano or the cello or conducting. We are making music together." He went on, a reminiscent smile warming his face. "But in this case it is true that I am the composer, and he has to do what I say. If I say 'erescendo' he has to make a erescendo. And if I mark the music to be played at 76, he has to play it at that tempo."

As he looked back over the crowded days of recalled, in a pleasurable compliment. "He only asked me once, just once, if he could do something that I had not indicated. He asked if it would be all right at one point to give a little rubato. And of course it was."

Then a reporter asked Rostropovich if he could say how he felt about the music of Leonard Berstein. "I have an emotional reaction." he began. "I love very much the music of Bernstein. And I am so proud - three world premieres by Bernstein and played by my orchestra. This is a great step for music. Bernstein is a complete musician, one of the greatest in the world. I think perhaps a performer in music, which Bernstein is, loves more kinds of music than the composer - perhaps he knows and loves more kinds of styles than the composer. Bernstein is both composer and performer, and therefore, he has great love for many kinds of music."

While Rostropovich had never conducted any music in anything like the kind of syncopated, big, often brash style that Bernstein makes so exciting, he had no difficulty moving into the new field with speed authority. At one point in the suite from the film music for "On the Waterfront," the conductor stopped the orchestra and turned to the violins. "More vibrato," he ordered. "Molto vibrato, much more. In melody, it must be like . . ." there followed a characteristic Rostropovich pause while the conductor searched for the colorful simile with which he loves to give the musicians an idea of just what kind of sound he wants. Then it came out: "like hot hamburger. At end of melody, okay like cold hamburger. But now, much vibrato."

At one point during the press conference Rostropovich riveted his attention on Bernstein as the composer was saying that he would like, during the rest of his life, to write a real American kind of opera. "Perhaps," he daydreamed, "with these same singers who are singing in "Songfest, if they could all free themselves from their millions of engagements, we could go off together somewhere and spend a month together, for perhaps 10 hours every day, and out of that, evolve an opera scenario. It would be wonderful.

Rostropovich could stand it no longer. He turned to Bernstein and said impulsively, "I make myself candidate to conduct you opera." Which seemed to please the composer-conductor greatly.

The fact that both men play the piano came up when Bernstein was asked about chamber music. "That is exactly what the Meditations are," he pointed out. "The orchestra is reduced, and the cello plays with them; at times there are only violas and cellos and basses, no violins. That is chamber music." Asked if he would like to play chamber music with cellist Rostropovich, he answered in a flash: "I would love to. I would like to start by playing both Brahmus sonatas for cello and piano, then five by Beethoven, then trios and quintets. But I would have to find the time to practice the piano."

This prompted another listerner to ask Bernstein if he had heard Rostropovich play the piano. "Indeed, I have," he said "I was so jealous." Perhaps the answer to their intimate rapport came when they were asked how long they had known each other. "Since we were born," said Rostropovich, "all our lives." "And before," added Bernstein. Which is the way things go when two great musicians have the time to get together to work, to play, to talk.

Rostropovich being one of the world's most persuasive musicians, perhaps it will happen agains some time in Washington when there is a need for two conductors, two pianists, a composer and a cellist, but only enough money to engage two musicians.