Alice and Clarice, the Rainer Twins, are playing shift-the-smile.

The smile is the strangest thing about their strange performance. It is not two smiles, but one. Alice and Clarice are seated at the piano, dressed in long blue lacy gowns, playing lacy trills, while that single girlish brightening oscillates between them. One smile, two women.

More than 200 sets of twins, almost all of them identicals, are cheering for the Rainers, and not only for the Rainers, but for something even grander, for twininity itself.

There is a stutter in the air, a joy in duplication, a rich redoubled weirdness. Two hundred sets of twins, their strangeness here not strange at all, are wallowing in twinship.

Certain rules apply at these annual conventions of the International Twins Association. The rules aren't written down--this is, after all, a friendly, family-type party--but the rules are followed nonetheless: Twins must dress identically, male twins must wear exactly the same T-shirts or exactly the same ties; female twins must wear the same earrings, the same dresses. Twins should wear their name tags. Twins should walk in pairs. They should come to meals together, and do their dining side by side.

Alice and Clarice, Gene and Dean, Mildred and Myrtle, Arlene and Ilene, Lollie and Polly, Rudolph and Ralph--all their lives they've had to bear that which-one-of-you-is-which stuff, but here it doesn't matter. All weekend they have wandered two-by-two through the sweltering streets of Charleston, 200 sets of twins happily ignoring--no, actually enjoying--the gawking of the singletons.

No one is making any money out of them. They've all paid their own way. The registration fee is only $15. They have come for one good reason. Here, for once, each pair of twins is a sort of individual. Here, and maybe only here, people locked in twinship can look around and feel, well, just like everybody else.

"I always say we," says Clarice.

The Rainer Twins are standing in the lobby of the Sheraton Charleston waiting for the bus that will take them on the city tour. They are giggling mischievously. At some unspoken signal, they pick up Viva and Vita, their two apricot toy poodles, and shove the uncomplaining dogs into a pair of Velcro-sealed shoulder bags. "We don't want to be individuals," says Alice. "If one runs to the grocery store, we feel strange. We've never been separated. We never seem to argue. We always, always agree. We're together 24 hours a day. We live in the same house. We play on the same piano. We still sleep in the same bed."

"Isn't this fun?" says Clarice. "It is so nice to be with others who, well, understand."

Don Dotson of Kearney, Neb., is attending the convention--alone for the first time. Dan couldn't make it. Dan's in Colorado. Don and Dan came every year from 1974 to 1977--and each year won first prize for looking most alike. They came again in 1981, and again they won.

"We have trophies at home this high," says Don. "I kind of feel out of place without him. Why did I come? For friendship. You meet two people at once, and you know they've experienced the same things in life that you have. Lots of people say it's like having a brother--but it's not. Last week we broke up with our girlfriends. We didn't know we were going to, but we did it at the same time. I'm a UPS driver. My brother works on the railroad. We're both in transportation. We never fought. But we wrestled, competitively, in high school. We were good. Because we practiced all the time at home. We were so good we wrestled for the state championship in 1968. He whipped me 6 to nothing. I didn't want to beat him. I really didn't."

Only certain types of hard-core twins regularly attend these annual conventions. Out there in the real world, two-eggs-two-sperm fraternal twins outnumber the identicals perhaps two-to-one. Not in the International Twins Association. Eight of 10 pairs present appear to be the product of a single fertilized egg that, for reasons not yet understood, divided in the womb.

Their similarites are stunning. They look alike, they walk alike, they complete each other's sentences. Time and time again they say they never fight or argue or find trouble in their we-ness. One fraternal twin--who'd never dreamed of hanging out with hordes of other twins, who still recalls, in detail, the money-fights, the fist-fights, the screams and splintered doors of his adolescence--hears such claims so often that he eventually believes. Many of these twins share more than a birthday. They share entire lives.

There is, if truth be told, something over-sweet, something syrupy abroad here. It is nice, no doubt, to have a buddy, a comrade, a companion, a friend with whom you can explore a lifetime of shared memories. But twininity is not pure joy. Like most other sorts of love, it has its hate component. Twins who've had to flip the coin time and time again to see who gets the bigger half of the steak, know that sharing, after all, means that one gets less. They remember, too, the subtle jabs from twin to twin that strangers never noticed, and the blurring of identity, of freedom and self-confidence that twinship seems to breed. Adolescence is tough enough for everyone. For twins it's even tougher. The twin who as a teen-ager is driven by his hormones, or the pressures of his peers, toward the dark shoals of romance, knows a special sort of tearing. In order to prepare himself for an intimacy that's new, he first must hack at one that's older. Yet here at this convention, such pains are discussed rarely. Twins walk down the halls as if in perfect bliss, sometimes hand-in-hand.

Only rarely does one notice a harsh condemning glance, a note of slight embarrassment, a gesture of suspicion. Dawn and Denny Turner of Toronto are only 10 years old, long-haired, blue-eyed and lovely. They seem rare among their peers. They shoot each other frowns. Despite their pink hair ribbons and their pink party dresses, they fidget in their chairs. Could it be that those two girls, alone here, are troubled by the twinship bit, refuse to buy it whole?

"Do they fight?" says their mother Donna. "They fight like demons. They actually slug each other. If one is hurt, or threatened by someone else, they'll run immediately to the rescue. But leave them to themselves, and you should hear them nag."

Dawn is asked who wins their fights. "Denny," she replies. Denny, asked the same question, has another answer. "We both do." But Dawn says Denny wins the fights, she is told. Denny meets her sister's glance and there receives permission. "Yeah, I guess I do."

Mary and Margy Cowden, 70, of 3601 Connecticut Ave. NW., say they've never had a disagreeable moment.

"The ESP is great," says Margy. "We often buy the same gift, or pick exactly the same birthday card. When we're out together, we know exactly when it's time to go home." Both twins are wearing bright blue T-shirts bearing the legend "Twinsburg, Ohio." The twin convention there last month was three times as big as this one; 729 pairs attended. Twinsburg is named for Aaron and Moses Wilcox, business partners who lived next door to one another, married sisters, had the same number of children, fell sick on the same morning, died on the same day--Sept. 25, 1827--and are buried there, in Twinsburg, in a common grave.

We've never, ever fought," says Ralph Flori of Rhode Island. His brother, Ed, agrees. "When we were five or six, we used to dream each other's dreams. We've always been a team."

The Flori twins, the co-vice presidents of the International Twins Association, not only look alike. They look a lot like Elvis. Their speech is aw-shucks Elvis speech, they each have Elvis spit-curls, and their bronze belt buckles bear the singer's name.

"Elvis had a twin," says Ralph, "a stillborn brother, Jesse."

Elvis is only one of history's famous twins. Helen Kirk of Galveston, who calls herself "Miss Helen: Supertwin Statistician," has assembeled a 29-page list of others: Montgomery Clift, Robin and Maurice Gibb (of the Bee Gees), John V. Lindsay, John and Chuck Panozzo (of Styx), Ed Sullivan, Billy Dee Williams, Henry Morgan, Everett Dirksen, the late Shah of Iran, Jim Thorpe, Mario Andretti, Eleanor McGovern.

The Floris, both enthralled, are sitting in the lobby of the Sheraton Charleston, talking about Elvis with Jane Lazenby Ventenella, the Jane of Jane and Jean, the Lazenby Twins of Memphis, who actually stayed at Graceland while Elvis was alive. "Our second record," she says, "was about Elvis's dead twin. We called it 'Unknown Brother.' " She leans across the sofa. Seriously and softly, she begins to sing.

I have an unknown brother,

Unknown to you and I.

He is a way up yonder,

Up yonder in the sky . . .

When Jean went into labor with her first child, says Jane, "I felt the pain so bad, I hollered."

That sort of telepathic linkage is not at all uncommon among the twins attending this 49th convention. "I felt that sort of twin-pain for the first time in February," says Donald Keith of Reston. "It was weird--an electric shooting pain, I felt it in the groin. It lasted just a few seconds. I didn't know what it was until I called my brother, and discovered he'd pulled a muscle at exactly the same time."

Keith, one of the founders of the Chicago-based Center for the Study of Multiple Gestation, says that "twins who tell you there is no downside to their twinship are probably concealing it. Every twin, in my opinion, bears some psychic scars--from too much closeness, too much competition, too many comparisons."

Keith says he's "obsessed" with twins. "I come to this convention to feed my soul, my psyche, to be with my own people."

His people are not shy. Most are upbeat all the time. A few, it's true, will tell you, often whispering in corners, that many sets of twins that come to scenes like this only rarely marry, and when they do it turns out that one twin or the other is soon to be divorced. Many of the twins here, not all of them, but many, are exhibitionists of sorts. They speak fondly of the Toni twins and of the Rinso twins as well. The Rainer Twins, professional "duettists," are envied by their peers. The Dotson twins have thought of modeling in New York, for Wrigley's Doublemint, for instance. No one here attending this 49th convention of the ITA is intent on hiding out.

On Sunday afternoon they gather in the hotel's ballroom for the Twin Judging Contest, the highlight of the weekend. It's like a beauty contest, except alikeness and not beauty is the goal for which they strive.

Two by two they climb the stage and smile at their peers. More than 20 pairs win prizes.

The crowd applauds each showing. The losers take their fate in stride, and then embrace the winners, who shriek and sometimes even cry as if Bert Parks had just announced their names at Atlantic City.

Don Dotson, out of the running this year, is watching from the back of the room. Suddenly he notices a half-familiar face. "Hey, Joe," he says delighted.

"It's not Joe," says Joe. "It's Jeff."