What do you do when you have two great cellists on stage and only one cello? Last night in the Tawes Theatre at the University of Maryland the solution was simple. Mstislav Rostropovich went to the piano, while Leonard Rose played his cello in Faure''s "Ele'gie," a tribute by two great cellists to a third: the late Pablo Casals.

It was the only encore which followed a recital that earned Rose a standing ovation from an audience made up mostly of fellow cellists.

"Slava, how shall we play?" Rose asked. "Too fast or too slow?" "Something in between," replied Rostropovich, and proceeded to play a very accomplished piano part, slightly hesitant and muted at the beginning, but vigorous and dominant when the music called for such treatment. There could hardly have been a better beginning for a meeting that is already historic: the first American Cello Congress.

"This is a unique occasion," said Rose, a master of understatement in words as he is in music. He came out on the stage with a microphone, after taking several bows at the end of his recital.

"I would like to thank you very much for the warmth of your reception," he said, referring to the extreme heat and humidity in the hall. Most performers would worry about playing for an audience of their peers. Rose, who does not have enough peers to fill a jury box, let alone the Tawes, seemed to take a special delight in playing for an audience of colleagues. His performance, under less than ideal conditions, was almost a textbook model not only of techniques but of sheer musicianship. The program ranged from the classic dialogue of Beethoven's Sonata in C, Op. 102, No. 1, to the terse, subtly shifting phrases and accents of Debussy's Sonata. There was also a strong dose of sentimentality in the Fantasy Pieces, Op. 73, of Robert Schumann and even more in the same composer's Adagio and Allegro, and Ernest Bloch's "Prayer: From Jewish Life." The sentimental passages, in particular, were played with a restraint that enormously strengthened the music. A brilliant conclusion was provided by the Feuermann transcription of Chopin's "Introduction and Polonaise Brillante."

In all of these varied styles, Rose was a complete master, ranging through an enormous variety of tones, accents and subtle shifts of dynamics and tempo with total assurance, delicacy of touch and an eloquence approaching that of a human voice. Pianist Andrew Wolf was an accompanist fully worthy of his partnership with this great cellist.

Rostropovich, seated in the audience, had been one of the first to rise for the standing ovation. In his remarks after the ovation, Rose mentioned that Rostropovich was present and that the Cello Congress had been Rostropovich's idea. "He is the greatest cellist of our generation," said Rose. Rostropovich also showed that he's a pretty good pianist. If the occasion had been unique before, the encore moved it into a completely new dimension.