Two great choreographers join forces for 'Brahms/Handel' . . .

It could be called ''post-modernism comes to the ballet.'' The New York City Ballet's new ''Brahms/Handel,'' (which premiered here June 7) is an exciting departure - bringing, for the first time, Twyla Tharp's avant-garde style to the company George Balanchine built. But this admirable project, mixing the choreographic talents of two of the finest dance creators alive today - Tharp and Jerome Robbins - has not been entirely successful.

When co-ballet master Robbins invited Tharp to develop this piece with him, he must have known the considerable challenge they faced: blending the stylized but still formal movement of Robbins with the carefree, angular, and highly individualized movement of Tharp.

It was no easy task. Unfortunately - as was clear at the premiere two weeks ago - they did not succeed, at least for two-thirds of the ballet.

The dragging flexed foot, the exaggerated run, the quick lunges and flicks of the hand - all trademarks of the Tharp style - stuck out in sharp contrast to Robbins's more stately pace, classical arms and legs, and slightly jazzy feel. It was interesting to watch a dance experiment being tried before your eyes, but it lacked feeling, depth, and most of all, a clear identity.

Half the company, dressed in blue and led by Merrill Ashley and Ib Anderson, seemed to take on the role of the more formal and restrained, working their way through pas de deux and traditional turns, leaps, partnering. The other half, dressed in bright green and led by Maria Calegari and Bart Cook, displayed more of Tharp's modern flair.

This split style reached its highest effect when, as Ashley and Cook led the ''blues'' through some beautiful straight-line partnering, Calegari, standing atop four men's shoulders like some sort of Greek goddess, entered with the ''greens.'' Slowly, like a fallen idol, Calegari tumbled head over heels to the ground. The audience didn't know whether to laugh - or try to comprehend the wonders taking place on stage. Adding to the confusion, the split between halves was not clearly maintained, as ''blues'' and ''greens'' ended up dancing elements of both choreographer's styles.

At times, one or two couples presented some gorgeous pas de deux work, when suddenly six or eight more dancers would appear behind them, offering some equally interesting dancing - usually in Tharp's modern style, with flexed feet, off-center movement, and a playful mood.

All these scenes are typical of post-modernist dance - the mixing of contrasting styles, the fragmentation of concentration - but there weren't enough of them to clearly guide the viewer into the new frame of consciousness post-modernism demands, a consciousness which accepts the fact that you're supposed to focus on the sum total of shapes and forms being presented instead of on one couple or one grouping. Lacking clear direction, the ballet seemed unsure whether it wanted to leap into the post-modern or just casually mix the contrasting styles of Tharp and Robbins.

The lead dancers encapsulated this split identity. Anderson and Ashley executed the dance well but seemed out of touch with the Tharpian flair that pervaded the piece. They moved without that focused energy which rivets one's attention on a great dancer (though both have often proved capable of this in the past).

On the other hand, Calegari and Cook infused their dancing with power and resolve. In one series of solos, they each made sizzling runs and turns, throwing hands and arms this way and that, leaping, turning, and then striking a dashing pose. Ashley and Anderson, however - though dancing some of the same choreography - did not compel or attract like the other two. Perhaps this is intentional, since each couple seemed to represent a certain style overall; but if they were being asked to dance both styles, shouldn't they have been equally as convincing?

I'm not sure, but just when all seemed hopelessly unclear, Tharp and Robbins found their meshing point - and the last one-third of the ballet sang. Duos, quartets, sextets came dashing on, and at awe-inspiring speed moved from pirouettes to flashing hands and kicking legs to throwing and dragging each other across stage. The sharp contrasts of style melted away, and one felt the ballet moving relentlessly forward, its self-imposed contradictions gone.

This led to a marvelous finish: As the two lead couples turned and faced front, arms extended, the rest of the company slid up behind them in a bunch. Then, in a splendid burst of color, each woman was thrust skyward and carried outward, blues to the right, greens to the left. It was beautiful and innovative.

Perhaps, given the choreographers' ability to find their meshing point later on, the earlier section of the ballet just needs trimming and focusing. This is a striking and provocative new contribution to the City Ballet's repertory, and it deserves the further refining needed to make it a truly superb dance.

The New York City Ballet will be presenting ''Brahms/Handel,'' other new works, plus dances from the repertory during the company's performances at the Saratoga Arts Festival, July 3-21, in Saratoga, N.Y.