They went to school together, they live together, they work in the same place and, come this summer, they will compete in the same weight category at the Commonwealth Games.
The wrestling duo could even be forced to face each other in the first round, depending on the draw.
Fiercely competitive, this would be a nightmare scenario for the 44-year-olds who live in Hamilton.
"They don't do seeding in wrestling, you go to the weigh-in and you pick a number out of a hat and that determines where you are in the draw," explains Fiona.
"So basically we could draw each other in the first fight. That wouldn't be good."
But, Donna adds: "The good thing is we know one of us is going to win it and one of us is going to the next round but you don't want to fight each other at a major championships like that. Our first international compe-tition for wrestling was in Madrid and that happened, we drew each other in the first fight. We couldn't believe it."
Whether it is on the mat or in the gym, competing against your body double leaves no room for error.
On first glance it is impossible to tell who is who, only the red and blue vests give it away.
Over the years they have followed the same training patterns, eaten the same food and lived in the same places.
Together they have travelled to compe-titions all over the world, from Cairo to the Grand Canyon.
At the age of 11 they took up judo in their home town of Campbel-town, Kintyre, and later competed at the Commonwealth Games in Auckland and Manchester.
Donna was the first to win a medal when she took bronze in 1990, followed by Fiona who achieved the same in 2002.
BUT when they were training at the Palace of Art in Bellahouston Park they tried wrestling to pick up new moves to add to their judo technique.
After a few sessions, the pair were hooked, and with the invitation to join an emerging Scottish squad, they began training full-time for New Delhi 2010.
Just newcomers to the sport at that stage, neither won a medal in India but the compe-tition cemented their love of the sport.
Speaking at the Sir Chris Hoy Velodrome, where they train in the centre of the track, Fiona said: "After Delhi we weren't sure if we would keep competing but when we heard Glasgow was getting the Games we thought 'well we have to'.
"I know the pressure we felt at Manchester. Even though it was below the Border it still felt like a home games because of the crowd, you got a lot of people travelling down.
"Wrestling is such a big sport in India that, in New Delhi, the noise when an Indian wrestler was fighting was immense. I know Glasgow will be like that.
"I have felt it in Glasgow - what a buzz there is, people are talk-ing about the Games and people know what the Games are now who didn't know."
Having both taken a five-month sabbatical from their jobs at the leisure centre in Hamilton, Fiona and Donna spend all day, every day thinking about wrestling.
And they always push each other harder to improve.
Although they are relatively old as far as athletes go, the twins are showing no signs of slowing down.
They are still improv-ing and meeting fitness targets and neither see age as a barrier.
But funding is becoming harder to secure and, although it is not on the cards at the moment, they have plans to dedicate their retirement to develop-ing the sport and getting more women involved.
They regularly visit primary schools across the West of Scotland where many of the pupils are surprised to find the wrestlers aren't wearing capes or hitting each other with chairs like they do in popular WWF style wrestling.
But Fiona and Donna laugh off that common misconception and teach youngsters about the techniques of freestyle wrestling.
"Eventually when we do stop competing, not that we are setting any dates at the moment, we will either do judo or wrestling training and development, and try to get more females into the sport because a lot of them don't know what it is and are a bit afraid of it," says Fiona.
BUT for now, their focus is on the Games and both sisters have their eyes set on medals.
"I know I am going to have to fight out of my skin to get a medal because India is strong, Canada is strong and Nigeria is stronger than anybody thought.
"It will go down to the draw as well but I will be going out there to win a medal."
Donna added: "We know what it is like to go home with a medal and we know what it is like to go home without one and it is 100 times better going home with a medal. "