Treated as medical guinea pigs they were burnt, frozen, starved, electrocuted, kept forcibly awake and injected with radioactive and other toxic substances in the name of "science".
The twins were institutionalised for much of their lives but their horrific story has now been revealed in full for the first time by journalist Juliet Butler.
Ms Butler befriended Masha and Dasha and said despite sharing the same genetics, horrific childhood and all of their lives the same lower body - the sisters had wildly different personalities.
One was a cruel, domineering "psychopath" who was "emotionally abusive" to her caring, empath sister who remained gentle and kind and longed for a normal life.
Ms Butler, who met them in 1988 and became friends with them, told MailOnline: "I've no doubt at all that Masha was a psychopath - she ticked all the boxes.
"Dasha was in an emotionally abusive relationship - similar to the situation some people find themselves in with a partner.
"But while those people have a chance to leave, Dasha physically couldn't.
"Masha denied Dasha everything she ever longed for - a chance of love, a relationship with their mother, a job and even what she wanted most: a separate body."
Ms Butler, who has written a novel based on their lives, The Less You Know The Sounder You Sleep, has provided fascinating insights into the tragic pair that reignite the nature versus nurture debate.
They were kept hidden from the public in a children's ward for eight years before being moved to boarding school for children with motor-impairment in southern Russia.
As adults, the women had blocked out the trauma of their earlier childhood, according to Ms Butler.
"It was myself who told Dasha and Masha that they had been subject to these cruel experiments from birth until the age of six, when I found out about them,
"They said they couldn't remember. They could recall only happy things - such as the time a nurse brought them a toy.
"Masha got angry while Dasha just said "It wasn't their fault, they were just doing their jobs". That's a typical reaction of sweet Dasha."
She reveals how Masha was unable to consume alcohol so forced her sister to drink - so they could both get drunk.
"They had their own hearts and lungs but shared a blood supply, so when one drank alcohol they could both get drunk," she explained.
"But Masha was unable to because of a gag reflex. And despite Dasha detesting alcohol she forced her to drink anyway.'
With medical advances, many doctors offered over the years to separate the girls, but were declined - by Masha.
Ms Butler recalled visiting the sisters in the late Nineties with a letter from a British surgeon who specialised in separating conjoined twins, offering to help them.
"Dasha glanced across at Masha, her eyes full of hope, but Masha, looking straight ahead, immediately said "nyet". And that, as Dasha would say, was that.'
And when later life they found their birth mother Yekaterina Krivoshlyapova in 1985, again Masha decided after four years to cut off the connection - against Dasha's wishes. And sadly, when the girls met their two brothers they were rejected by them because of their looks.
Dasha wanted a job and tried a job putting rubber bulbs on pipettes - but Masha was not interested in changing her life and would smoke and read magazines.
Even after death - Dasha was loyal
After making an appeal on television, the sisters moved to a Home for Veterans of Labour with greatly improved living conditions.
Ms Butler says that despite their sad lives, the sisters were an inspiration.
"I wanted to write the book to show people how sweet Dasha was. And it kind of is a tale of triumph over tragedy.
"In the end I noticed how Dasha stood up to Masha and put her in her place more.
"Despite the toxic dynamic, at the end of the day they had been through a lot together and they had each other. They clearly loved each other deeply."
Then on April 17, 2003, Masha died of a heart attack - even then Dasha refused separation, perhaps out of her own need to stay close to her sister, or out of loyalty. Dasha was taken to hospital and died another 17 hours later due to blood poisoning from the toxic by-products of Masha's decomposing body.
The Less You Know the Sounder You Sleep by Juliet Butler, published by 4th Estate, is out now.
