Image caption Abby (l) and Brittany (r) Hensel and friends

Abby and Brittany Hensel are conjoined twins determined to live the normal, active life of outgoing 20-somethings anywhere. They have been to university, they travel, they have jobs. But how easy is it for two people to inhabit one body?

Like most 23-year-olds Abby and Brittany Hensel love spending time with their friends, going on holiday, driving, playing sport such as volleyball and living life to the full.

The identical, conjoined twins from Minnesota, in the United States, have graduated from Bethel University and are setting out on their career as primary school teachers with an emphasis on maths.

Although they have two teaching licences, there is one practical difference when it comes to the finances.

"Obviously right away we understand that we are going to get one salary because we're doing the job of one person," says Abby.

"As maybe experience comes in we'd like to negotiate a little bit, considering we have two degrees and because we are able to give two different perspectives or teach in two different ways."

"One can be teaching and one can be monitoring and answering questions," says Brittany. "So in that sense we can do more than one person."

Their friend Cari Jo Hohncke has always admired the sisters' teamwork.

"They are two different girls, but yet they are able to work together to do the basic functions that I do every day that I take for granted," says Hohncke.

Media playback is unsupported on your device Media caption Taking a road trip with conjoined twins Abby and Brittany Hensel

The twins know each other so well that they often say the same things or finish each other's sentences, and are supportive and understanding of the other in all aspects of life.

Conjoined twins Conjoined twins develop from a single fertilised egg and are therefore always identical and of the same sex

It is unknown why the embryo does not complete the process of separating into identical twins

Records over the past 500 years have shown around 600 sets of conjoined twins survived infancy - more than 70% of these were female

Despite medical advancements, surgical separation is still very rare today Find out more about the condition

With two sets of lungs, two hearts, two stomachs, one liver, one large intestine and one reproductive system, they have learned from a young age to co-ordinate their body, with Abby controlling the right hand side and Brittany the left.

There is a difference in height and at 5ft 2in (1.57m) Abby is taller than her sister Brittany who is 4ft 10in (1.47m). As their two legs are different lengths, Brittany has to stand on tip toe, on her leg, to ensure they maintain their balance.

They have had to learn to reach compromises on everything from what food they eat to their social life and even the clothes they wear.

"We definitely have different styles," says Abby. "Brittany's a lot more like neutrals and pearls and stuff like that and I would rather have it be more fun and bright and colourful."

Media playback is unsupported on your device Media caption What is clothes shopping like for conjoined twins?

While Abby is seen as the "outspoken" sister and will always win the argument about what they are going to wear, Brittany says her twin is also much more of a "homebody," whereas she prefers going out.

There are other differences too. Brittany is scared of heights, whereas Abby is not. Abby is interested in maths and science, while Brittany prefers the arts.

They also respond differently to coffee. After a few cups Brittany's heart rate increases, but Abby is not affected.

Image caption Abby and Brittany Hensel tried high tea on holiday in London

And they have different body temperatures.

"I can be a totally different temperature than Brittany would be," says Abby, "and a lot of times our hands are different temperatures, so I get super-hot way faster."

Why conjoined twins were once known as 'Siamese twins' Conjoined twins Eng and Chang Bunker were born on 11 May 1811 in Siam, now called Thailand. They were joined at the chest and although the knowledge was not available at the time, it would have been possible to separate the twins using today's medical techniques. At the age of 17 they joined Scottish merchant Robert Hunter's world tour exhibiting themselves. Their fame led to the term "Siamese twins" being coined to describe conjoined twins. In 1839 they became American citizens and took the surname Bunker. Four years later they married two sisters Adelaide and Sarah Ann Yates. Chang had 10 children and Eng 11. After suffering severe bronchitis, Chang died in January 1874 and Eng shortly afterwards. Their liver is on display at the Mutter Museum in Philadelphia.

Despite having a normal family and social life, studying and working like any other young women, they do face some additional problems.

For example, they have to put up with speculation about their private life - something they prefer not to discuss. The twins deny a rumour that Brittany has become engaged, describing it as a "dumb joke".

Travelling to a new country with friends on holiday is also not as straightforward for conjoined twins. They have two passports, but one ticket as they only take up one seat on the aeroplane. However they also have to be on their guard and more aware of entering crowded or confined spaces because members of the public will often try to take unwelcomed photographs.

As a close friend of the twins Erin Junkans says you always need to be alert because you never quite know how people are going to react or what they are going to say.

"I want to make sure that they're safe and that they're not completely exposed, definitely just standing in the way of pictures, just always keeping an eye on what's going on and just how the girls are reacting to the crowds," says Junkans.

"Sometimes if they get more overwhelmed then… we just need to get away from [the] area for a little bit, but they amaze me at their ability to just shake it off and keep seeing what we are there to see."

Conjoined twins are very rare - it is thought one in every 200,000 births - and around 40-60% of these births are delivered stillborn. Female siblings tend to have a better survival rate than male siblings.

Any operation to separate conjoined twins is a highly complex and dangerous process. It was a risk that Abby and Brittany's parents did not want to take for fear that one of the twins might not have survived the surgery or have the same quality of life they do now.

Image caption Abby and Brittany Hensel celebrating their 22nd birthday with a party

With possibly fewer than 12 adult pairs of conjoined twins across the world today, Abby and Brittany Hensel are defying the odds. Their mother, Patty Hensel, says her hopes and aspirations for her daughters' future are just the same as anyone else's.

Why do a TV documentary? "We decided to do a series because it's fun and you can see a normal day-to-day life of who we are and what we do," says Brittany. Abby says: "It's just going to be a fun thing for everyone to say, 'Oh they do do normal things and they have great friends and they're always doing something. "It's normal what we do - our life isn't any different from yours."

"Like every mum would hope for, you want them to be successful and to be happy and healthy as they're being successful and that's what I want," says Patty Hensel.

As they embark on their working life together, the twins aim to take things day by day and do not tend to look forward to where they will be or what they will be doing in 10 years' time.

With their position as teachers they have become role models for children academically, but also in their attitude to life, overcoming any challenges.

"I don't think there's anything that they won't try or something that they couldn't be able to do if they really wanted to," says Paul Good, principal of the school where Abby and Brittany work.

"To bring that to children, especially kids who might be struggling, that's very special, that's learnt through lived example."