When they were born, their mother was advised to wrap them up and leave them facing each other to mimic their young lives in utero.
For 'mirror' twins Hugo and Ross Turner, from Devon, it was the beginning of a bond that has seen them forge a life of two-man global expeditions - all documented on social media - together.
In fact, so inseparable are the Turners that they have been studied by scientists trying to learn more about the genetics of twins, reports the Daily Mail.
It could all have been very different for the handsome, curly-haired brothers, who've just become the first adventurers to reach the South American Pole of Inaccessibility - deemed one of the most difficult points to reach in the Southern Hemisphere.
A devastating diving accident at the age of just 17 left Hugo with a broken neck and a promising rugby career in tatters.
For the first time, the twins began to carve a different path, with Ross continuing to play rugby across south west England while his brother Hugo - forced to undergo an operation to reconstruct his neck - spent 18 months rehabilitating.
Hugo says it was the hardest of times: "It was very difficult, especially when you're 17-18 and all you want to do is run around and play sport but there was a slight bit of me that was just relieved that I could walk and I wasn't paralysed. It didn't stop me from being part of the teams at school - I became chief water boy."
With professional sport off limits, the pair looked at how they could make their dream of becoming adventurers a reality instead - allowing them to use their bond to help them reach some of the planet's most remote places. The trust between them has only been further strengthened by their often harrowing adventures, explains Ross.
"We've definitely experienced a new level of relationship because of the expeditions. You do heavily rely on each other.
"Our last expedition to the Green Pole, when we crossed the Greenland ice cap, your twin becomes your lifeline.
"If you fall down a crevasse, if you break a leg or if you go into some sort of shock from a broken arm...your twin is then 100 per cent saving your life and I think that's when it becomes more than a relationship - it becomes a bond that we can't really explain."
"It's a weird feeling when you put your life in someone's hands - and I don't think many people experience it."
They've volunteered to let scientists monitor their reactions on expeditions to see how differently they react to the same situations, hoping to help shed some light into mirror twins.
The phenomenon results when a fertilized egg splits into two 9 to 12 days later than expected and can see twins displaying reversed asymmetry, eg one twin will be left-handed, the other right-handed.
Explains Hugo: "We've always looked at our expeditions and thought how can we use them more wisely.
"Over time, we've become more engrossed, more interested in it [the science]. On our latest expedition, Ross felt the altitude a lot more than I did and I felt the heat far more than Ross did.
It's quite fascinating because we do expect to react in the same way - they don't actually know whether it's nature or nurture - a lot of it is diet and health and lifestyle.
He adds: "We have exactly the same lifestyle and we're still working with them to focus on these subtle things - it's personally very interesting as to why we're different."
With seven more trips to more poles of inaccessibility in the pipeline over the next five years, the pair know they're sometimes making it hard for the ones they leave behind.
While Hugo has a girlfriend, Ross is currently single but both say they appreciate the worry they cause families and friends when they put themselves in such dangerous scenarios.
A television career seems like the next natural step and they're social media savvy, looking at ways to monetise their ever-growing 'Turner Twins' profile.
Says Ross: "The next natural progression for us is to go into TV. I think everything we've done to date is fairly honest. We wouldn't do anything if they were trying to put us into a format that wasn't honest.
He admits that "the twin thing is the biggest USP we have; we don't need to be risky, we can just be honest, we can do our own thing."
There's also a sense that even if the television deal doesn't come, they're happy living their dream.
Ross explains: "So many people don't live their dream - you've got 40 or 50 years to work and if you're not enjoying it then you're just wasting life.
"A lot of our friends are jealous [of what we do]. From the outside in, it looks quite dreamy but from the inside out there's lots to do - you make your own luck."
THE RARE CASE OF MIRROR TWINS
The term mirror twins, refers to those who have the same physical features but opposite asymmetric features.
This particular type of twinning may occur because the fertilized egg separates later than usual.
The split occurs between 7 to 12 days after fertilization and the two identical halves develop into separate individuals who are genetically identical. A fertilized egg that splits after 12 days would likely result in conjoined twins.
Though scientists continue to study the phenomenon of multiple births and why some factors happen in the womb, much about the creation of this type of twinning is still unknown and the probability of having identical twins seems to be random.
Source: Twin Pregnancy and Beyond