When Two of a Kind Beats Everything : LIVES OF THE TWINS <i> by Rosamond Smith (Simon & Schuster: $16.95; 236 pp.) </i>

Since the beginning of time, people have been fascinated with twins. In classical mythology, Zeus raped Leda, a woman in the form of a swan, and Castor and Polydeukes were born. There are the biblical twins, Jacob and Esau, Pharez and Zarath. There are Siamese twins, humans physically joined--at the hip, the breastbone, the head. Primitive tribes in Australia and Africa traditionally murdered one or even both twins at birth--and sometimes the mother.

The fascination continues to today. I don’t think there is a person alive who hasn’t fantasized what it would be like to have a twin--someone who not only looks like you but knows you, knows what you’re thinking, what’s inside your soul. And all the wicked tricks you could play. Just in the past few months there has been a series on twins on “The Today Show,” on a Newsweek cover and now a new book by Rosamond Smith called “Lives of the Twins.”

In this novel, we meet Molly Marks, 30, who, on the night she moves into the seventh-floor apartment in the Greenwood Towers in Lindesfarne, Conn., with her lover and former psychotherapist, Jonathan McEwan, learns that he has a twin brother, an identical twin brother, a “mirror image” twin.

Jonathan asks Molly if she knows what is meant by “mirror image.” Molly mentions that, “If the two of you stand side by side you look as if your mirroring each other?”

Jonathan replies, “If we stand side by side our faces appear slightly dissimilar but if one of us is viewed in a mirror while the other stands beside him facing the observer you can see that our faces are very nearly the same. Of course, we can tell the difference . . . . In any case,” adds Jonathan, “he isn’t very important in my life.”

It turns out that Jonathan has not spoken to his twin brother, James, in nearly 15 years. And that James is also a psychotherapist (“it was a mockery of me”), that Jonathan is right-handed and James is left-handed, that their hair “whorls” in opposite directions, that James gets cavities on the left side of his jaw, Jonathan on the right, that the vision in James’ left eye is weaker than in his right, the opposite being true for Jonathan.

When they were 10 years old, Jonathan fractured his right ankle in a bicycle accident. A few weeks later James fell off his bicycle and fractured his left ankle. “Of course, that was an accident. I mean--accidents,” continues Jonathan. “There must have been a number of similar coincidences in our life--I mean our lives. But they are equally minor. They mean nothing. It’s probably the case that James and I have less in common than ordinary brothers, but since we were born twins, other people expect us to be alike--there’s that tendency toward exaggeration, myth making. People like to romanticize twins, particularly identical twins, because they have the same genes--.”

Not very important indeed.

It isn’t long before we get to the heart of the matter. “I will never forgive James--no one can expect it,” says Jonathan that night. “If they knew what he did, what he involved me in--.”

This is too much of a challenge for Molly Marks, who has just quit her sixth job, has a detailed rose tattoo on her left arm near the shoulder, dark, wavy waist-length hair, and “is skilled at gauging the effects her words have on others and rarely needs to pay attention to the words themselves.”

Without telling Jonathan, the man she loves, the man she hopes to marry and the man she hopes will father her child, she sets out to find James.

Molly creates her own twin as it were when she assumes the name Holly Hawkes, locates James in New York and proceeds to become his patient and lover. She discovers that the terrible secret between Jonathan/James involves a woman. Molly/Holly sets out to find her, does so and discovers the terrible secret. The book ends with Molly/Holly, gun in hand facing the two twin brothers, desperate to kill James, and not knowing which one he is.

“Lives of the Twins” is quite fascinating and compelling, but there is also something slightly distracting about it. It’s as if a very bright schoolgirl spent hours in the stacks of UCLA’s Powell Library, studying everything she could about twins and is now telling us what she learned. (Actually the stacks in Princeton’s Firestone might be more accurate.) In New York publishing circles, the gossip is that Rosamond Smith is really Joyce Carol Oates, Smith/Oates describes Molly/Holly as a “pretty young woman who suspects at times she’s a little too clever for her own good; too clever by half, as a former lover wryly noted.”

That’s kind of the feeling one gets reading this book--that it’s too clever--by about half.