Meet Tatiana and Krista Hogan of Vernon, British Columbia, Canada -- 4-year-olds conjoined at the head, known as craniopagus twins.
"They are very interconnected. They share a lot of things normal conjoined twins don't. They have the special ability to see what each other's seeing through each other's eyes," Felicia Simms, the girls' mother, said in a television interview with ABC News.
The girls' pediatric neurosurgeon, Dr. Doug Cochrane, explained, "They have connections amongst blood vessels, both arteries and veins. But they also have this connection between what's called the thalamus, and between the thalami, one in each to the other, so there's actually a bridge of neural tissue in these twins, which actually makes them quite unique."
The sisters can also feel what each other is doing, and according to Simms, most likely hear what each other is thinking, as well.
Despite the unusual connection, both girls are happy and playful, though they do occasionally disagree on which direction to go, resulting in minor skirmishes.
Of the two, Krista has grown a bit larger than Tatiana. But it's the smaller Tatiana who supports both, as her heart beats for both.
"Tatiana is more of the laid-back one," Simms said. "She'll do anything her sister tells her to. She doesn't usually fight with anybody unless they step on her toes one too many times. Krista is more of the aggressor. I call her my little bully."
Craniopagus twins occur in just 1 in 2.5 million births, oftentimes stillborn. And while others have survived, like Tatiana and Krista, they typically do not fully share one brain.
Take Lori and George (born Dori) Schappell, for example. At 49, the sisters are the oldest living craniopagus twins and enjoy very separate personalities. They do, however, share 30 percent of their brain tissue, most of which is in the frontal lobe.
George, who changed her name from Dori to Reba before choosing George in 2007, suffers from an abnormal development of the spinal cord, leaving her 4 inches shorter than Lori. In order to move around, she sits upon a wheeled stool, which Lori pushes around. The two face opposite directions.
The sisters spent the first 24 years of their lives in an institution in their hometown of Reading, Pa., because a court determined their parents could not care for them properly. As adults, they were finally allowed to leave.
In her book, "Conjoined Twins," Christine Quigley says Lori attended college and has worked as a linen aide in a hospital delivery room. When the hospital expressed concern for George's well being while her sister was on the clock, she offered to sign a waiver against any liability.
George once aspired to be a physician but eventually pursued a career as a country western singer. She has performed throughout the United States and earned an L.A. Music Award for Best New Country Artist in 1997. Fortunately for Lori, she's a fan of her sister's music and buys tickets for each concert (and gets the best seat every time).
The Schappell sisters are content with their situation. As George told The New York Times in a 1997 article:
"There are good days and bad days -- so what? This is what we know. We don't hate it. We live it every day. I don't sit around questioning it, or asking myself what I could do differently if I were separated."
And they have no wishes to face the risks of separation.
"Why would you want to do that? For all the money in China, why? You'd be ruining two lives in the process," she told the Times.
Separating adult craniopagus twins has been attempted only once, in the case of Laden and Laleh Bijani, of Iran, in 2003 at the age of 29. The two had enjoyed academic success studying law and wished to lead physically separate lives, despite the risks. Sadly, both died during the procedure.
However, separations have been successful with babies and toddlers. In November 2009, 2-year-olds Trishna and Krishna, connected at the tops of their skulls, were separated at the Royal Children's Hospital in Melbourne, Australia. The girls had been rescued from an orphanage in Bangladesh prior to the procedure.
As for Tatiana and Krista, their bond leaves no option for separation. But their family has met the Schappells, who've given them hope that the girls can develop their own individuality and grow up to do whatever it is they might wish to do.