When they first set up their design business DED in the early 90s, they made a name for themselves designing album covers for local bands like Babybird and punk doyenne Siouxsie.
Over the years their work has diversified and they now run DED from a gallery and event space in Sheffield, but their latest project has been the most challenging, the most ambitious and the most testing of the twins' relationship.
Together with Nik's wife Emma, the pair started searching for a site to build their homes about five years ago. They already lived just a few doors down from each other and were in and out of each other's houses several times a day.
So in some ways it was a small step to amp up the shared living arrangements and find a plot of land where they could literally step out from one doorstep onto the other.
Although many would find the prospect of building one house (let alone two) from scratch rather daunting, Nik and Jon grew up thinking it was quite normal to camp out in a caravan while the builders toiled next door.
Their parents have built three houses over the years, including the family home in Loxley where Nik, Jon and their older brother Jeremy grew up. “Jon and I used to sit with Dad while he was working on the plans, drawing our own dream houses. We haven't ended up keeping the secret passageways, the helicopter pad, or the slides, but in some ways our dreams never changed,” says Nik.
And the twins' father David has been instrumental in supporting the project. “He's a hero,” says Jon. “He inspired us as kids, and now to be on this project at 76, fitting all the electrics, doing all sorts – the inspiration just keeps coming.”
Living on-site in a rented cottage enabled Nik and Jon to commit all their spare time and energy to the project.
Borrowing a digger to excavate the land around the pond, lugging 70,000 kilograms of concrete bricks up makeshift ladders and overseeing every detail, the pair practically slept in their hard hats for 18 months.
The result is stunning. The epitome of industrial chic, the houses also conjure up a sense of rural idyll, despite the former rough-and-readiness of the setting.
The 80-metre dam, which once provided power for the corn mill, has now been completely cleaned up. These days the tranquil waters are a haven for kingfishers, owls and herons, with striking Yellow Flag irises crowding the pond's edge.
In the evening, moonlight mingles with the lights of neighbouring houses to illuminate the water. It's a romantic view for Jon and his partner Ali Jarjour. The same view is the picturesque backdrop to Nik and Emma's impressive open-plan kitchen-diner. It's flooded with light from the 7.5m by 3m floor-to-ceiling windows, which were lifted into place by a heavy-duty crane.
Mature oak and ash trees overlook the modern-industrial homes, contrasting with the steel frames and wraparound black corrugated cladding. Both houses are perched on stilts to maximise the views, with the original water wheel spindle carefully preserved underneath. “We wanted to pay homage to the fact this was an industrial space,” says Jon. The exposed pipes and untreated concrete flooring are softened with plants, vintage leather sofas and velvet chairs.
With the twins having eight children between them, ranging in age from 12 to 21, the houses need to function practically as well.
Each home has five bedrooms and three bathrooms. The family dog, Wilf, has the run of a downstairs playroom and teenagers' den, which also includes a small kitchen.
Jon and Ali's bedroom, with its free-standing bath, is on the top floor with valley views. The couple met during the build, prompting Jon to re-draw his plans to incorporate two extra bedrooms.
This last minute re-draft wasn't the only delay. As with all major house-building projects, Nik and Jon encountered many other obstacles along the way.
The build was already more than six months overdue when their flooring was laid – which then took 90 days to fully dry, delaying everything further. Nik and Jon also had difficulties with contractors letting them down.
“Trying to focus on this at the same time as running our business has been more stressful than we thought it would be,” admits Jon.
Nik agrees: “We thought because we'd always worked together that we'd be fine, but there were some dark days. Even then though, when we did disagree, we'd be fine the next day. After all, you can't argue with the mirror.”
Although the build went over-budget, with a final cost of around £610,000 for both houses, Nik and Jon say the Grand Design team were impressed by what they saw.
Now both families are living in the Corn Yard, after four years in rented accommodation, they can finally enjoy the fruits of their labour.
For Emma, it's all about the light and space: “After the dinginess of the cottage, it's like living amongst the treetops. I love it.”
It's almost 40 years since Nik and Jon started designing their dream home as kids. They might have lost the slides and helicopter pad, but does the rest live up to their vision – and was it worth it?
“Totally,” says Nik. “When we were getting the house ready for filming I sort of forgot we were really going to be living in it. It all felt a bit surreal. But now we're in, it's fantastic. We can look at what we've created and say, ‘we did that, together'. It feels like quite an achievement.”
Nik and Jon appear will appear in Grand Designs on Channel 4 next Wednesday at 9pm.