It would not be an optical illusion if, in the final meters of the New York City Triathlon on Sunday, two chiseled 5-foot-3 women wearing yellow-and-black Livestrong stretch suits appeared in striking distance of each other after swimming 1,500 meters, cycling 40 kilometers and embarking on a 10-kilometer run.

The one known for her game face is the defending champion, Rebeccah Wassner. Her ever-smiling body double is her fraternal twin, Laurel. Rebeccah has always beaten Laurel, but the gap is narrowing.

“I’d love to see it come down to a sprint finish,” their coach, Cliff English, said.

Each has been USA Triathlon’s elite rookie athlete of the year. The two have shared podiums. They have lived, worked and trained in New York City for most of the past decade, but Rebeccah had a four-year head start as a pro.

Growing up near Gaithersburg, Md., the Wassners pursued different sports and hung out with what their father called “the nerd herd.”

Rebeccah went on to run N.C.A.A. Division I track and cross-country for Mount St. Mary’s in Emmitsburg, Md., and Laurel earned a swimming scholarship as a walk-on at George Washington. They majored in accounting and graduated in 1997.

Rebeccah stayed in Washington to become an accountant for Deloitte & Touche and pursue a running career.

“I always knew that whatever I was doing, she could easily do,” Rebeccah said. But for eight years, Laurel did not have an equal chance.

In 1998, Laurel, then 23, was in Central Park when she felt three bumps on the left side of her neck. It turned out to be Stage 2 Hodgkin’s disease.

Rebeccah transferred to New York City, found an apartment with Laurel on West 12th Street and continued to work and run hard while her twin faced six months of chemotherapy.

“I tried to be the one who was, ‘O.K., everything’s normal,’ ” Rebeccah said. “ ‘Everything’s going to turn out O.K.’ ”

But it was not normal. In the waiting room at the hospital, Laurel would say: “Everyone here is old. No one here looks like me.”

She tried to maintain her identity as an athlete by running on a treadmill at the gym. It was almost effective.

“People stared at me because I was bald and still running faster than them,” Laurel said, but once, someone sarcastically called her Sinead O’Connor. She did not respond but thought, “I was like, ‘You have no idea.’ ”

On Tuesdays, Laurel’s chemotherapy days, Rebeccah did her hard runs with a women’s team in Central Park. She worked to exhaustion so she could share Laurel’s fatigue.

Three days after Laurel’s final treatment, Rebeccah ran her first marathon, in London, and her twin — pale and skinny, with no eyebrows — wore a baseball cap on the sideline. That date, April 18, 1999, had sustained Laurel through treatment.

“It was my goal,” she said.

Rebeccah placed 36th in less than three hours but preferred a sport that she could do more than once or twice a year. So in 2002, she dusted off her bike and spent weekends riding up Route 9W north of New York City to master cycling.

The pool at Chelsea Piers, the roads of New York and flexible work hours at Amy’s Bread also enabled Rebeccah, at 28, to win the 2003 national amateur triathlon age group championships. She turned professional a year later, and after she finished third at events in New York and in Chicago, USA Triathlon named her the 2004 elite rookie of the year.

But the next year, she was sidelined by a pelvic stress fracture and began to convert Laurel, who had worked in the photo department at Men’s Journal and Fast Company, to triathlon.

“I knew she could beat me in the swim, but she wasn’t 100 percent confident in her health,” Rebeccah said. “She’d complain after running 12 minutes. I’d say if you make it to 20 minutes, you’ll be fine.”

Laurel finally grasped the magnitude of the sport in Lausanne, Switzerland, where she and her younger sister, Sarah, watched Rebeccah place 52nd at the 2006 world championships.

“As soon as I got there, I knew I wanted to be part of it,” Laurel said.

One month later, Laurel said, doctors told her: “You’re done; you’re free. You never have to go back to the hospital.”

She said, “It was a huge weight off my shoulders.”

About a year later, she turned pro.

In one of Laurel’s first pro races, in Philadelphia, she started to beat Rebeccah in the swim, and Rebeccah remembered thinking: “This is actually happening. Everything is coming together.” She added: “It was definitely emotional. Good thing I was underwater.”

Rebeccah won, Laurel was third, and with four other podium finishes that year, Laurel was named the 2008 elite rookie of the year.

Then last year in New York City, Rebeccah won convincingly and Laurel placed sixth.

In May, Rebeccah could not vie for her third consecutive title in the Columbia Triathlon in Maryland because of a torn plantar fascia in her left foot, but Laurel ran to victory.

Laurel not only kept the title in the family, she said: “I was finally able to do something that made everyone happy instead of really sad or stressed out. I had projectile tears at the end of the race. My sisters did, too.”

Then last weekend in Minneapolis, Rebeccah came back from her injury and placed second, finishing with a huge grin.

“I never smile during a race,” she said. “I can’t believe I let my guard down, but I was so happy to be in it.”

Laurel, too, has let down her guard. She only recently began talking openly about her cancer.

The twins, now 34, represent Livestrong and plan to establish a postcollegiate grant for a young person with cancer.

“I always felt I should help someone out,” Laurel said.

At the same time, the Wassners say they look toward qualifying for the 2012 Olympic trials.

Rebeccah placed 10th at the 2008 trials, but Laurel would have what English, their coach, called “the innocence of not knowing” what to expect.

If the United States earns three women’s berths, he said, “It might be tough to topple Sarah Haskins and Laura Bennett, but that third spot is wide open.”

Until then, triathlon fans could get used to seeing double.