PEOPLE tend to confuse the two Valentinos, but they couldn't be more different. Valentino Garavani, the Roman couturier, is dressmaker to Jacqueline Onassis and others of the international jet set and is a leading figure in the world of high fashion.
Mario Valentino, on the other hand, is one of the most important of the Milan ready-to-wear designers, whose collections are shown twice a year in that northern Italian city. The Valentino collection, made almost entirely of leather , never fails to bring rounds of applause, and last week's showing of fall-winter clothes for next year was no exception.
Born the son of a Neapolitan shoemaker, Mario Valentino is not so much a designer but a true Naples artisan, who has brought his leather craft to dizzy international heights. Like another internationally famous shoemaker, Salvatore Ferragamo, who began his career as a cobbler's apprentice in a small village in the Naples area, Valentino began his trade in his father's workshop in the heart of Naples. And like Ferragamo, he began his climb to fame by selling his shoes to America.
''Already 25 years ago I was traveling to the States several times a year,'' says this unpretentious family man. He also began his own line of purses and bags and a few clothes 15 years ago.
''The clothes were simply regular leather coats then; you needed no design for those,'' he says. But his wife, Bianca, was interested in designing and experimenting with leather, so together they brought their leather fantasies to the world of fashion.
The Valentinos now have a collection of 19th-century weaving looms, which Bianca started collecting to use for experimenting with their leather ''fabrics.''
The leather starts out as sheepskin, lamb pelts, goatskin, or deerskin. Pelts are stripped, cleaned, and tanned until the leather becomes velvet soft. Then, with the aid of heat, stretching, ironing, and cutting, it is woven, pleated, and textured to look more like cloth, such as tweed or linen, than leather.
Six years ago, a longstanding friendship with ready-to-wear fashion mogul Giorgio Armani led Valentino to work on leather as a clothes fabric to be taken seriously, not simply as a chic luxury. Armani, both a shrewd businessman and a gifted designer, recognized Valentino as ''a superb technician'' and began using his leather in more and more daring design ventures.
Finally the two maestros hit the limelight in the October 1981 showing of their spring-summer collections, featuring skirts, pants, and soft suede tops in riotous colors and basket-weave jackets in brilliant oranges, eye-catching purples, and emerald greens.
Later the two designers adapted their ''fabrics'' to last year's spring-summer collection. For Armani's oversize urchin-look coats and jackets, they used suede that looked and felt like velvet and an irregularly rippled fine leather that resembled crepe with a stiff lining.
After six years the Armani-Valentino partnership has broken up. This year's fall-winter collection, shown last month, was designed for the first time by Gianni Versace, a southern Italian designer who has made his name and fortune in Milan.
The clothes in this year's collection are made from finely woven fettuccine-like strips on a linen base, hand woven on the family looms. The clothes will sell for between $800 to $1,200 in New York and Washington stores.
Although the style of this new collection is a far cry from the casual, sophisticated tomboy look of Armani, the Neapolitan leather craftsman pronounces himself pleased with it. The styling is more formal, but still shows off the unique draft in the plissed full-sleeved leather blouson jackets and coats, which look as if they are made out of fine-grained soft elephant skin. Like all Valentino-crafted skins, they are light as a feather.
Jerkins are also made out of the softest black kid, pinched into a regular square pattern to look more like basket weave. Soft leather pants hang like gabardine, and the few colors used to contrast with this year's basic blacks, tans, coffees, and stony grays are brilliant emerald green, raspberry red, and cobalt blue.
Despite the acclaim their clothes get worldwide, the Valentinos have no ambition to widen their range of clothes, increase their output, and add mechanization to their cottage industry.
''I make enough money out of my shoes and bags,'' Valentino says. The business is big enough to employ his family - his wife, two sons, and a daughter - and 500 employees in two factories.
Their yearly output of clothes is about 10,000 pieces, much of it handcrafted. And while there are representatives of the Mario Valentino company in the United States, the business at home is coordinated by Valentino himself.
''I feel more like the conductor of a large orchestra,'' he says.
He has difficulty explaining what his particular job is because he has a hand in everything from administration (''My son does the main part of that; he has a degree in trade and economics,'' he says proudly) to the designing. ''I still design some of the shoes - they are what made me famous,'' he adds.
And, indeed, the twinkling multicolored Valentino pumps in leather, textured to look like fish scales or reptile skin or made from plain soft leather, were one of the first shoe collections that gave footwear a new life of its own rather than keeping it as a mere accessory that had to match or tone with clothes.
The growth of this family business into a concern that exports mainly to America, Switzerland, West Germany, and Britain also means that Valentino is continually on the move.
''I'm almost always traveling,'' he says with a hint of weariness. And nothing of the superficial sophistication of international jet-setters has worn off on this down-to-earth Neapolitan.
''I live in Naples, always have, always will,'' he says simply. ''I was born and bred there.''