When Holly Reich steps on stage and starts to sing, onlookers often express surprise that such a big sound can come from a person of such petite stature.

The young woman always smiles and responds in the same way: “Well, there were two of me to begin with.”

This brief acknowledgement is the only hint that Holly, 23, is a medical rarity — a surviving conjoined twin.

When she was less than a day old, Holly was separated from her sister Carly, who died six weeks later. Holly fought on for a year before being deemed strong enough to leave Great Ormond Street Hospital, London.

Now studying criminal justice at university and regularly performing, the only outward sign of her past is a scar from sternum to navel.

Ms Reich’s story, and her ability to lead a normal life, provides solace for the parents of Faith Williams, the conjoined twin given a 50-50 chance of survival after her sister died in Great Ormond Street this week following separation surgery.

She is one of a handful of people who understand what lies ahead for Faith: feelings of sadness over a relationship that never was; a sense of guilt – but also gratitude – that her sister died to save her; acceptance that some people will be cruel about her past; a determination to make the most of her life.

Speaking to The Times from her home in Texas, Ms Reich had some simple advice for Faith’s parents to pass on: “Losing someone, even though she’d never known her sister, will be difficult. But she should be grateful that she’s alive and be strong.

“And I would really want to say that she should never ever be ashamed of what’s happened, or of her scar. That is our battle wand.”

Holly and Carly were separated within hours of their birth on March 1, 1985. Doctors wanted to delay surgery until they were stronger but could not because Carly had a heart defect.

The twins shared an intestine, one large liver and a single sac around their hearts, and both nearly died from excessive bleeding during the operation. At the time, Carly was one of the world’s few surviving conjoined twins, overcoming the 1 in 200,000 birthing phenomenon.

This week history repeated itself at Great Ormond Street Hospital as doctors warned another anxious set of parents that their children might not survive surgery.

Just as in Ms Reich’s case, surgeons operated on Faith and Hope sooner than planned because the twins’ shared intestine was blocked.

Hope, so named because she was smaller, died on Tuesday after 11 hours of surgery. The pair had been joined from breastbone to navel.

Last night Laura Williams, 18, and her husband, Aled, 28, kept a vigil by Faith as doctors gave warning that the nine-day-old child remained “very sick”.

Ms Reich was obviously too young to remember her time in hospital but becomes upset when she thinks of the turmoil endured by her parents, Gill and Ronnie.

The BBC made a documentary about the sisters and their operation, an historic event as they were the first conjoined twins to be operated on by Professor Lewis Spitz, the paediatric surgeon.

“I’ve seen it a few times,” she said. “Its hard to watch, I look at pictures of us and it makes me cry because we were just so tiny, surrounded by all those tubes. Then I just think about what my mum went through. It’s tough to watch.”

After her release from Great Ormond Street, Ms Reich her parents and a three-year-old sister, Lori, moved to Texas, near Houston.

She suffered some health problems in childhood “but as far as reconstructive surgery as I got older, there’s been nothing. I feel very blessed, the fact that for all purposes I’m a perfectly normal human being”.

She still grieves for her sister, and occasionally feels guilt over the fact that Carly died to save her, but she has “tried to be an optimist”.

“I often wonder in situations ‘What would Carly have done?’, but I try not to think about how fun it would be if she was here. I want to make sure that what I do in my life is something that she would be proud of. I think about her every day.”

While Ms Reich plays the guitar and regularly sings and performs at open mic nights, she wants to be a FBI profiler when she graduates next year.

She gains solace from the fact that her operation helped future babies in similar predicaments.

“Because of Carly and I, Professor Spitz was able to help a lot more twins. We were first and it was new. It’s an honour for me to know that we helped him to learn more.”

Despite her natural optimism, she is all too familiar with the cruelty of human nature.

“I have to be careful. Sometimes I want to shoot up my hand and tell people, but you have to protect yourself because that’s something very personal and people can be cruel when they don’t understand. Most people say its cool though. They are usually supportive and say ‘Wow I’ve never met a Siamese twin before’ .”

When she was fighting for her life in hospital, her mother used to dream that she would grow into a “perfectly healthy, normal young woman”.

“At first you’d count the hours. You couldn’t look too far ahead: it would be that they’re one day stronger than they were yesterday,” Mrs Reich said.

“Once she hit a year, and then two years, then she went to school, they were different milestones.

“Now she’s just a normal girl. There’s still milestones, but now they are just like anyone else’s life. As far as me worrying about whether she’s going to live, well, she has.

“She’s alive and she’s thriving.”

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