They play side by side, walk hand in hand to school and loveÂ dressing-up games â€“ the picture ofÂ happiness at seven years old. Yet these twin girls were born conjoined â€“ bonded at the spine with a question-mark over whether they could survive.
Thanks to British medical expertise, Mille and Signe (pronounced Milla and Seena) have gone from strength to strength after surgery to separate them at the age of three months.
From the moment they worked out how to stand at the age of 11 months, every step has been a landmark, every year a milestone. â€˜Weâ€™re special,â€™ says Mille, without further elaboration.
Long before they were born at the end of January 2005, doctors had identified that the girls were joined back-to-back in the womb, fused at the base of the spine.
The Mail told at the time how their father Simon Stephenson and his Danish fiancÃ©e Yane Christensen had to make the agonising decision either to terminate the pregnancy before 24 weeks, or risk giving birth to twins who might need lifetime care if they survived.
Later Yane was diagnosed with a life-threatening liver disorder that could have killed the babies in her womb. But the couple from north-west London placed their trust in the confidence of doctors who were convinced they could separate the girls without causing permanent disability.
For the first weeks after their caesarean birth, Mille and Signe shared everything, even a nappy. When one cried, both got a cuddle. When one slept, both had to lie in a single cot.
Yane fed both together on her lap. Then, after an eight-hour operation at Great Ormond Street Hospital â€“ only the second of its kind to be performed in this country â€“ the new parents were able to hold one twin each.
â€˜We found ourselves sitting as far apart as we could,â€™ Simon told me. â€˜Iâ€™m sure it was a subconscious way of showing off to ourselves that we could do that.â€™
Doctors told the parents immediately after the operation it had beenÂ â€˜completely successfulâ€™. Since then they have found nothing to suggest any long-term difficulties.
Inquisitive strangers often ask the twins which of them is older, and by how many minutes. They reply that they are the exactly the same age â€˜because we were born togetherâ€™.
Yane usually just smiles and shrugs her shoulders.
The twins are known to family and friends as the Moosies. Yane explains itâ€™s an Anglicised corruption of mus, the Danish for mouse.
‘â€˜They were so small when they were born, we joked that they looked like a couple of mice in their nest.’
Initially, each girlâ€™s back remained significantly curved, arching in the direction their spines were fused.
â€˜They curled up like bananas,â€™ said Simon, a 40-year-old art director for an advertising agency.
â€˜It was so pronounced, we couldnâ€™t fit them into a child seat in the car.
But one night we were sitting watching TV when one of them suddenly stood up. Thirty seconds later, the other one did the same.
‘From then on, the spine problem disappeared.â€™
Yane added: â€˜We had tried everything to straighten their backs, strapped them into all sorts of chairs and medical contraptions, but nothing worked.
In the end, they just did it for themselves. From 11 months old, they would stand up, wobble a bit, fall down â€“ then get up and do it again.â€™
It was a major landmark in the twinsâ€™ transformation from helpless babies to children who were clearly determined to get on with normal lives.
Much of it is chronicled in hundreds of photographs that Simon and Yane, a 43-year-old freelance illustrator, took to record their daughtersâ€™ progress.
The couple (the girls are nagging them to get round to marrying so they can be bridesmaids) are awareÂ of the debt they owe medical staff who delivered the twins, and the surgeons who operated to save them.
As Simon put it: â€˜You realise how good the NHS is when you have a really serious problem. I donâ€™t believe we could have been seen by better people.â€™
Next month he will run in the London Marathon to raise money for Great Ormond Street, and is donating proceeds from a collection of signed prints he is selling through a website.
The artwork he has created features cartoon style illustrations of various animals. One, of course, is a mus.