Appallingly exploitative and teeth-gnashingly cute, "Two of A Kind," the CBS movie at 9 tonight on Channel 9, nevertheless succeeds, by both hook and crook, at demolishing rational resistance and rendering a viewer helplessly lump-throated. It's puddle-over time, no matter how loudly one's better instincts may protest.

Robby Benson, 26, plays a mentally retarded young man who turns 21 as the film opens, and George Burns, 86, is his 81-year-old Grandpa, a former companion who had moved to Florida and now comes home in a state of seeming senility following the death of his wife. The film, simplistically written by James Sadwith and beautifully directed by Roger Young, shows how the young man and the old man bring each other out of alienation.

The film is a "GE Theater" presentation, as was last year's tearjerker "Bill," also about a retarded man and recently flattered with an undeserved Emmy or two. Apparently GE is on the outlook for scripts that depict the emotionally or mentally disadvantaged as elfin and adorable, as if they were pudgy, furry creatures in a Disney cartoon. These portrayals may demean rather than dignify such people, and in so doing, they demean everyone, including those who watch.

In addition, the film passes along the specious fiction that the happiest people in a society are those who are essentially oblivious to it. The Benson character is being trained for "usefulness" in what workshop teachers refer to as "the competitive world." The screenplay sneers at this, and suggests that none are so happy as those who don't quite know what's going on. Again, this kind of pseudo-sensitivity philosophizing insults those portrayed and those watching.

Benson hardly alleviates misgivings with his performance as Nolie Minor, who has been overprotected by his mother (Barbara Barrie) and largely ignored by his father (Cliff Robertson, the human furniture). Benson shuffles, sniffles and shows a lot of teeth, suggesting Jerry Lewis in "The Nutty Professor." Burns sits motionless through his early scenes, has his first line half-an-hour in, and brightens later, though never to the cloying, cuddly-codger state of "Oh God!" and other recent Burns turns.

Nolie is at first shocked by the sight of his beloved friend, Grandpa, sitting silent and all but lifeless in a wheelchair. But when they are alone together later, Grandpa says he has been faking the senility because life is less trouble that way. With the boy, however, he finds a kindred spirit, and the two break out of the nursing home in which the old man has been placed and have themselves a holiday with a borrowed car.

This includes what can safely be called the most moving game of miniature golf in screen history.

At its best, when all the stickiness is out of the way, the film celebrates the natural affinity between the very young and the very old, something that has a place in many memories as one of the exalted privileges of childhood. And there is something inspiringly assertive about Grandpa's declaration that "I'm gonna play a game of golf before I croak." This film is shameless, and in some of its patronizing toward the old and the retarded, it is shameful, but it presses certain emotional buttons and bells go off. You may hate yourself for responding, but it's probably better to surrender than to resist.