The Cheech & Chong comedy "Up in Smoke" and the Donny & Marie Osmond comedy "Goin' Coconuts" rely on dissimilar sources of humor and aim to amuse different segments of a youthful movie-going public. Nevertheless, they have certain esthetic ups and downs in common.

Both films, now in multiple engagements at several area theaters, turn out to be sputtering farcical launch vehicles for performers who reveal distinctively amusing personalities and skills. They may not joke about the same things or appeal to identical publics, but Cheech & Chong and Donny & Marie have attained a disarmingly expert level of comic rapport. They look naturally funny together and set one another off to advantage. What they need are better engineered pretexts for imposing on the good will of their potential movie audiences.

Neither movie places sufficient trust in the ability of its stars to carry the show. "Up in Smoke" gets off to an uproarious start when Cheech, a Hispanic family man fleeing the East Los Angeles barrio, picks up Chong, the black sheep of an upper middle-class household, on the Pacific Coast Highway. In a brilliant reenactment of what must be one of their definitive routines, these Furry Freak Brothers from opposite sides of town proceed to get acquainted over a joint the size of a blunderbuss muzzle. It's a new classic among comedy-team encourters: hilarious rapport at first toke.

Cheech & Chong began to break my own resistance to zonked-out facetiousness. Their characters reflect an exaggerated form of obliviousness that depends on the existence of a drug culture. At the same time their oblivion partakes of a traditional clownish innocence.They're stupid with drugs in the same respect that some clowns have been stupid with drink, lust or naivete.

It appears that Cheech & Chong might be capable of using this new state of mock-stupefaction to satirize some of the delusions peculiar to drug culture cognoscenti. They don't sentimentalize the drawing, slangy, bleary stooges they pretend to be. Frankly disreputable and simpleminded, their characters emerge from the pot smog inside Cheech's gaudily tacky set of wheels looking as archetypically dumb, innocent and indestructible as the numbskulls played by Laurel & Hardy.

The special nature of their oblivion may be best illustrated by Chong's attempt to bring down Cheech from a pot high at the approach of highway patrolmen. "Take these, man," he advises, offering a handful of capsules to his new crony. A second after Cheech has consumed this offering, Chong frowns and mutters, "Hey, don't take those man; I think I gave you some acid." Suddenly apprehensive, Cheech says, "I never had no acid, man. Chong tries to comfort him by taking the long view: "You better not be busy for about a month, man."

The goofball buoyancy of this introductory sequence doesn't survive the desultory effort to string out a plot. The focus shifts from the woozy but funny heroes to antagonists - a crazed Vietnam veteran played by Tom Skerritt and a relentless narc played by Stacy Keech - whose hostility looks pathological rather than harmoniously funny. Cheech & Chong are such original stooges that it may be a mistake to fall back on cliched antagonists as the principal butts of humor. Another technique must be found to sustain the stars for feature length while they drift about in a Magoo-like daze, miraculously surviving their own chemically induced detachment from reality.

After a brief, sensational bit by June Fairchild, who mimes the reaction of a wacko who mistakenly snorts a plate full of Ajax cleanser, "Up in Smoke" slowly but irreversibly nods off. The decline may not damage the movie with its intended audience, which is encouraged in the ads to enter feeling no pain. Nevertheless, Cheech & Chong could attain greater comic consistency without straightening out their uniquely bent sense of humor.

Lou Adler's direction can't compensate for the lumpiness in the scenario written by Cheech & Chong themselves. It's apparent that Adler is striving for easygoing, shaggy-dog wit, but the illusion itself seems to depend on the presence of his afably mangy comedians. When obliged to concentrate on secondary characters or exposition, the movie loses its offcenter espirit and veers toward complacent mockery of The Straights and tedious busy work.

As role models for the young, Donny & Marie Osmond are located at the opposite end of the spectrum from Cheech & Chong. Nevertheless, the Osmond kids have been cagy enough to perceive the comic possibilities in their own exuberant, shiny wholesomeness. They're so comfortable with straightness that they feel confident mocking it.

The plot of "Goin' Coconuts" is as negligible yet inhibiting as the plot of "Up in Smoke." It too overbalances by relying on the antics of would-be hilarious villains, rival gangs trying to retrieve a valued necklace that has been given to Marie by a courier for one of the gangs prior to boarding a plane from Los Angeles to Honolulu for a concert date, Donny & Marie are stalked by gang members scheming to snatch the bauble from her slender neck.

The movie has been deliberately formulated to mix ingredients calculated to please juvenile fans of the Osmonds. Along with a little Donny & Marie joshing-and-singing there's a perfunctory mystery plot, a batch of ridiculous bad guys, stunt sequences and Hawailan scenery, brightly photographed by Frank Phillips. The ingredients may be safe enough, but they're porportioned with an excess of caution, leaving the movie top-heavy on the expository end and tentative about exploiting its young stars.

It's probably just as well to err on the side of caution the first time around. After all, Donny & Marie are still very young and rather modestly talented. The worst thing their brain trust could do is entertain delusions of grandeur a la Joe Brooks. However, who can deny that Donny & Marie have more to offer the movies than Joe Brooks? There's a genuinely charming, perky element of humor in their brother-sister rivalry, which finds Marie adamantly refusing to be cowed by Donny's attemps to pull rank to her.

With luck and expert support Donny & Marie might be able to give this basic situation an extended movie-going ride. As a piece of professional light entertainment, "Goin' Coconuts" looks more presentable than most of the Disney comedies, Elvis Presley musicals, Frankie & Annette musicals or "Grease," an inflated imitation of the last two prototypes. However, if Donny & Marie have aspirations beyond this wading-pool level of screen exposure, they'll need to be extended more as both singers and comedians.

Director Howard Morris seems reluctant to let a song play without in terruption. He's always cutting away to pick up plot threads that might just as well wait until the stars had finished performing. This jittery approach to the musical numbers may reflect a lack of faith in the attention span of kids, but the practical effect is to undercut the performers themselves. I'm not crazy about Donny & Marie's overlacuered vocal style, but I'd prefer to watch any song performed from beginning to end without editing hitches.

It's also conceivable that an exceptional comedienne may be lurking behind the toothsome, baby-faced cuteness of Marie Osmond. Her round, diminutive configuration and wickedly gleaming saucer eyes recall the comediennes of the silent screen, and her independence of spirit is impressive to behold. When Donny goes into one of his patronizing big-brother speeches, she fixes him with a withering ironic gaze that no man would like to meet head-on. It's probably not in the cards, but something wonderful might happen if she found a character that could resolve the funny incongruity between her soft facade and wised-up outlook.