San Diego — On December 26, 1997, Janna and Nancy Sipes thought they were going to die. The two San Diego sisters were trapped through the night 18,500 feet up on Tanzania's Mount Kilimanjaro in a howling, freezing gale.

"That night was the longest of our 40 years," they wrote later. "We spent it oscillating between prayer and panic. Janna lay awake all night, unable to stop shivering. Unthinkable thoughts crept in. She began to think she wouldn't live until morning. Nancy too was entertaining thoughts of death, and she was already ill.

"Though it seems surreal to us now, even when we believed we might die on Kilimanjaro, we drew strength from each other. Dying without the other would have been a horror neither of us could bear, but we could face anything as long as we were together."

Living or dying together counted for more to Janna and Nancy than most siblings. They are identical twins, sisters who are the result of a single fertilized egg in their mother's womb that split in two. Unlike fraternal twins, who result from two sperm fertilizing two eggs, identical twins are genetically the same.

The two sisters, with doctorates in law and biology, had decided to climb the mountain to mark their 40th birthday. They survived the night and made it to the summit the next day, but their thoughts at that moment of crisis illuminated a truth they say belongs to all twins: no matter how the world tries to pull them apart, they are part of each other for life.

Now the two want the world to learn this too.

They have written a book about it called Dancing Naked in Front of the Fridge, and Other Lessons from Twins (published two weeks ago by FairWinds Press).

What the book shows is that twinship is at the same time wonderful and very tough, both on the twins and others who love them.

They draw the lessons largely from their own lives. The dream time, the ideal time for Nancy and Janna was when they were kids. They had been born four minutes apart. For their first years, they were never that long apart again. It was an idyllic intimacy. They were so close they often talked to each other in what they call "twinspeak," nonverbal exchanges. They experienced "twincidence," independently having the same thoughts at the same time, or literally feeling pain when the other was hurt. They were a sufficient universe unto themselves, even though they had two older sisters. "Once we got settled into our spots at bedtime -- we each had our own side of the bed -- we would entrust our treasured secrets and fanciful dreams to one another," they write. "Those innocent conversations would often turn into gales of laughter, causing a knock on the wall from our parents' adjoining room with a warning to go to sleep. This command usually resulted in our laughing even harder, forcing us to dig ourselves deeper under the covers in an attempt to muffle our giggles. For twins, every night is a slumber party!"

Then, with their introduction to school, they suffered a trauma that has largely steered the rest of their lives.

"In 1963, our parents were told that we would have to be in separate classrooms in the first grade," they write. "We had not spent more than a few moments apart in all six years of our lives. Now, suddenly and shockingly, we were separated. And we did not know why. What had we done to deserve this punishment?

"Our fear at being separated from each other during those early weeks of the first grade was indescribable. It was magnified only by our feelings of emptiness and loss. Even though we were surrounded by other children, deep inside, we still felt completely alone because we did not have each other.

"You can only imagine the horror for us every day in the lunchroom as we saw each other across the room, but were not allowed to sit together."

"Nancy was crying in what seemed to be slow motion," writes Janna. "I sat in my seat screaming in agony on the inside."

"School officials said that [together] we wouldn't learn to socialize and develop as individuals," Janna says when we meet at Nancy's Clairemont house. "Total bullshit! It was totally wrong. We think it was very traumatic on us because they did that. Some twins are ready [to separate] by the first grade. We were not. We were extremely close. We'd go to bed every night holding hands and touching noses and sleep that way. We couldn't separate. Then to hear, 'You're going over here, and you're going over here,' it just raised a bunch of abandonment issues."

The twins grew up and went to school in Texas. But it's no coincidence that after 40 years, the two live ten minutes apart here in Clairemont. Nancy's house, hidden behind a riot of purple Morning Glories, is filled with African and Asian masks, paintings, posters, and a fridge-load of bon mots. "No bird soars too high if he soars with his own wings (William Blake)." "A man's reach should always exceed his grasp (Robert Browning)." "To thine own self be true..." sits on a magnet on the fridge beside a wine rack full of bottles of Moët & Chandon champagne. "Life may not always be the party we hoped for, but while we're here, we should dance!"

"This is Nancy's house," says Janna. "She was always the untidy one."

Nancy is married, Janna's not. While Nancy's husband Stu is away at work, the two spend the day together, working on projects such as the book and a column for Twins magazine. I have to make a mental note that Nancy is the one with the triangular-shaped earrings on, though in the end it doesn't matter much. The two of them start and end each other's sentences and thoughts seamlessly.

"I was the dominant twin in the twinship," says Janna, who, you start to notice, does most of the talking. "I made the decisions from when we were little kids up through adolescence. 'Nancy, we need to leave at eight o'clock in order to get to school... Nancy, you need to do this, you need to do that.' Someone had to make the decisions. And it just fell out that I was the decision-maker for the twins. A twinship is going to be very chaotic, unless someone takes the lead. Though we would have these tremendous battles. Fisticuffs, fights, some hair-pulling. But we had to figure out a way to make the twinship work. And one way to make it work is you get mad real fast and you get glad real fast too. Because you spend literally 24 hours together, every day. It's part of the relationship tools. Just like in a marriage, you settle into roles."

"To the outside world, we really have this protective attitude toward one another," says Nancy. "I can say something bad about Janna, but no one else better even agree with me."

Adolescence gave them opportunities: "We switched on our boyfriends, and they couldn't tell -- until we kissed. And then they went, 'Yuk -- which one are you?!' " It also started pulling them apart. "Outside the relationship I found I was very shy, and not controlling and domineering," says Janna. "And Nancy out there made friends who looked up to her, and she made the decisions and she ran with the ball."

This was probably a good thing. Many twins, the writers say, have difficulty breaking out of the comfort of their twin relationship and forming their own individual "outside" friendships.

On the other hand, Nancy and Janna say that often their gregariousness and trust developed within their own relationship made them vulnerable in outside relationships. "Often, we've ended up sharing too much of ourselves too easily, too soon," they write in the book. "Trouble holding back in new relationships is a by-product of the twin 'trust-bank.' Twins have a tendency to seek in others the closeness they have with their twins, because their twinship is the gold standard for love."

And therein, say Nancy and Janna, lies the real, underlying problem.

"As we grew up, our twinship did not. When we were together, no matter what our age, we would revert back to our childhood twin selves: Janna as dominant caregiver; Nancy as passive follower. We were unable to separate psychologically because our first separation [in first grade] had been so traumatic. We were trapped by an underlying fear of losing each other."

And Janna feared losing her caregiver role. "I remember when we first lived apart from each other [as college students]," says Nancy, "Janna's last words to me were, 'You'll never be able to balance your checkbook!' And 'You can't make it without me!' "

But living independently, Nancy earned a Ph.D. in biology, and Janna a doctor of jurisprudence degree. The two led successful, separate lives, although according to Nancy, "the phone bills were enormous."

Then in 1991, at age 33, despite the fact that Nancy had shifted to San Diego, gotten a job, and married, Janna couldn't stay separated any longer. "I'd told Nancy that I felt we had to live together again to help me let go of my dependence on our twinship," writes Janna in the book. "My feeling of being incomplete without her would not be healed until I could understand us as individuals."

It was a disaster.

"Within minutes of entering Nancy's house, I surveyed it and drew conclusions on the rearrangements that would be required to make it 'home.' I had pushed a well-worn button for Nancy. Childhood memories of my control over her came rushing back. For the first time ever [Nancy] attacked me with a viciousness of a lioness protecting her cubs. 'This is my house! You are a guest! I like it the way it's decorated, and you will not be allowed to change anything!'"

It was the moment that forced both sisters to "aggressively examine" their relationship. For the next seven years they worked on "detaching from our twinship." They even interviewed 17 other sets of twins. The book documents the results.

And yet today, here in Nancy's house, the two appear closer than ever. In their account of their adventure on Mount Kilimanjaro, the fact that Nancy's husband Stu was there didn't even rate a mention. Apparently, Nancy's fear was of losing her sister, not her husband.

"Poor Stu!" she says. They both laugh. "He perpetually goes through life feeling he's in second place. Janna and I, now that we've quit our jobs and are writing together, we spend all day together. And Stu will come home and she'll call me, and we'll be on the phone talking. And he'll say, 'What else could you possibly have to talk to her about?' It's not like we're ever going to run out of things to talk about. We just are each other's favorite plaything. Stu realizes that he can never be as close to me as Janna, and that's not a comfortable place for a significant other. You're used to being the number-one person in your mate's life. Stu and I do go on vacations together, and I try very hard to have weekends just for us. But it doesn't change the fact that Janna's still the most important. I'm closest to her."

On the way out, we're looking up at an Indonesian three-face mask. The center one is being squeezed in by the others. It looks as if it's screaming in agony. "Stu!" say the twins. "Stuck in the middle! We're in an eternal triangle. He says all the time, 'I married twins.' "

One in 80 live births in the U.S. are twins. A third of those are identical. The rates are increasing. The twins say they hope the book will help this growing number of floundering parents.

"The best thing my mom has said in years was when she finished reading the book. She put it down and looked at us and said, 'I wished I'd had this in 1957.' That was the year we were born," says Janna. "No one knew anything about multiples then."

I ask if they've considered having children of their own. They're unanimous. "If we could be guaranteed we'd have twins, we probably would have both had children," says Nancy. "...because I can't imagine," says Janna, finishing Nancy's sentence, "how one person goes through life by themselves, without this constant soul mate."

Since this interview, Nancy says she has left her marriage and now lives with Janna.