Like many twins they have very different personalities and tastes - even more so now they are teenagers. Abigail, the feisty, stubborn one, likes orange juice for breakfast while Brittany, the joker of the family, will touch only milk.
Abigail loves pink and all things girly but Brittany prefers purple, multi-coloured hair and wearing unusual hats, and now they have turned 16 they love to experiment with makeup and clothes and giggle about which boys they like.
But that is where the similarity with other teenage twins end. For Abigail and Brittany Hensel are conjoined, sharing one body fused at the torso. Each controls just one side of the body, and yet remarkably this has not prevented them leading a full, active and happy life.
Displaying an astonishing co- ordination which has stunned doctors, they play the piano -with Abigail taking the right-hand parts and Brittany the left - and enjoy sports such as bowling, volleyball, cycling, softball and swimming.
And on their 16th birthday they passed their driving test; a mind-boggling feat of teamwork with each twin using one arm to control the steering wheel. Brittany explains: 'Abby does the pedals and the gear shifter. I take over the blinkers and the lights. But she likes driving faster than me.'
Which as their mother Patty, a registered nurse, concedes, could prove a problem. 'I don't know what would happen if they got pulled over for speeding. Would they each get a ticket or just Abby because it's her foot on the accelerator?'
The Daily Mail first introduced the Hensel twins ten years ago, when they were six years old, and now a decade on these remarkable pictures reveal the dramatic progress they have made as they approach adulthood.
The incredible bond which was so evident when they were children has strengthened year by year into one which neither twin ever wants to see broken. As Brittany says: 'We don't know any other way.'
The Hensels are believed to be one of only four sets of dicephalus twins in history to survive infancy and, to mark their 16th birthday last year, the girls allowed the cameras into their fiercely guarded private world to share this milestone in their lives.
'Believe me, we are totally different people,' says Brittany, to which Abigail adds: 'I'm more into like pink and girly and Brittany is more not into pink...we take turns. One day Brittany will pick the outfit and the next day I will pick the outfit.'
It is not unknown, however, for the twins to go out in the specially made top with two different necklines - to reflect their unique tastes - and leggings with each leg a contrasting colour and a different shoe on each foot.
When the Hensel twins were born on March 7, 1990 in Minnesota in the United States, doctors warned their parents Patty, a registered nurse, and Mike, a carpenter and landscaper, that they were unlikely to survive the night.
Just one set of twins in every 40,000 is born connected in some way to each other and only 1 per cent of those survive beyond the first year. The Hensel girls are the rarest form of conjoined twins, the result of a single fertilised egg which failed to separate properly in the womb.
They have two spines (which join at the pelvis), two hearts, two oesophagi, two stomachs, three kidneys, two gall bladders, four lungs (two of which are joined), one liver, one ribcage, a shared circulatory system and partially shared nervous systems. From the waist down all organs, including the intestine, bladder and reproductive organs, are shared.
Yet Patty, 46 and Mike, 47, never once considered having the twins separated, through fear that one or both might die or be left with such severe disabilities their quality of life would be compromised - although today Patty sometimes wonders if she made the right decision given the advances in medical techniques.
But she knows that to separate the twins would mean they could no longer enjoy all the activities they love. They would each have just one arm and one leg and be confined to a wheelchair. It is a choice neither she nor the girls are prepared to make while they remain in good health.
Patty had no idea she was carrying twins until the birth at the local hospital where she worked; she was heavily sedated after the delivery as doctors realised the seriousness of the situation and whisked the twins to a larger hospital in the nearest big city.
'The paediatrician said my babies were together but they had two heads,' she recalled. 'It was blunt but completely accurate. From the first time we saw them, we thought they were beautiful.
I kissed Abigail and then Brittany and gave them a hug. It's like that every time I pick them up from school, two kisses and one hug for the most beautiful children in the world.'
Both Mike and Patty's families have lived in a small midwestern farming community of 300 people for generationsand it is here where they have brought up the twins and younger brother Dakota, 14, and sister Morgan, 12, away from the media spotlight.
Although Brittany is more susceptible to colds and has twice suffered pneumonia, the twins are in good health despite a series of operations.
In infancy a third undeveloped arm was removed from their chest and aged 12 they underwent surgery to correct scoliosis - curvature of the spine - and expand their chest cavity to prevent future breathing difficulties.
They attend a private church school and are popular with their friends, who treat them no differently from anyone else. Only when the family ventures outside this close-knit community does the curiosity of strangers have the potential to wound.
Once Patty heard a child at a swimming pool ask his mother if she had seen the little girl with two heads. 'We have talked about that with Abigail and Brittany,' she said.
'When children ask the girls if they have two heads, they say they don't but that each has their own head. That's what we have encouraged them to do, to develop their own individuality as much as possible.'
That has meant buying two seats every time they go to the cinema - even though only one will be used - separate meals and two different birthday cakes with candles each year. If one of the twins misbehaves, Patty and Mike are careful to scold the individual responsible - even if the other has been dragged unavoidably into the misdeed.
Yet, while the twins have developed their own tastes in food, drink, clothes and separate personalities, their body works as one - although they have different urges to eat and sleep.
When they eat they have separate plates one of them holds the fork and the other the knife to cut the food, and then take turns to put the meal in each other's mouth.
Sometimes it is simply easier for them to share a meal and they will hold a hamburger in their hands from which Abigail will take a bite and then Brittany. Similarly, they take it in turns when they do their hair and make-up.