The pretty four-year-old twins have two separate bodies but share the same brain.
The girls have a conjoined thalamus, the part of the brain that sends physical sensations and motor functions to the cerebral cortex, allowing them to hear each other's thoughts and see through each other's eyes.
But it wasn't until their proud mum Felicia Simms saw them playing that she discovered that they could see through each other's eyes.
She said: 'When they are playing, one of the girls will reach over and grab something from her sister's side and know exactly where it is without possibly being able to see it.
'It's absolutely awesome to watch them sometimes because there's no way she can see the toy she is reaching for and it's just incredible.' The girls also seem to experience each other's emotions.
'If one of the girls is hurt, the other can feel it and if you discipline one the other will also cry.'
The girls, from Vernon, British Columbia, Canada,have been receiving constant medical care since they were born.
Paediatric neurosurgeon Doug Cochrane, who has looked after them from birth, confirmed they can see through each other's eyes.
He said: 'The twins are sharing signals from the other twin's visual field.
'One twin may see what the other twin does, as the brain of one of the girls receives electronic impulses from the retina of the opposite twin.'
Felicia and her childhood sweetheart, Brendan 26, were unaware their daughters were conjoined until she was five months pregnant.
She underwent an ultrasound scan which diagnosed the twins as having a condition known as Craniopagus.
Felicia said: 'The doctors told me: 'The twins are joined together. Further tests are needed, but at the moment we know they are linked in some way'.
'I felt like someone had hit me with a sledgehammer.It all seemed unreal, surreal really, like I was living in a dream and about to wake up.
'But seeing them on the ultrasound, it suddenly all became very real. Whatever happened though, I knew I was going to give them the best possible shot at life that I could.'
Incredibly rare, only eight cases have ever been documented and just three of those have survived the birth.
When the girls were born by caesarean section they weighed just six-and-a-half pounds.
The girls had separate bodies but were joined just above the ear and Krista was pumping and filtering the blood through her kidneys.
'She is basically keeping both of them alive,' Felicia says.
'Their brain is knitted together so any attempt to split it could be fatal to one or both of them.'
Felicia, who is also mum to Rosa, Christopher, and Shaylee, said she never considered a termination but says she received hate mail over the years since the twins' birth.
'They called the girls freaks,' she explains. 'The nameless letters said that I should never have had them.
'But I didn't pay them much heed. After all if they weren't prepared to give up their names, then why should I care.'
The family agrees Krista is the bossy one of the pair, while Tatiana is more laid-back and follows Krista wherever she wants to go.
Felicia said: 'Tatiana is definitely the more laid back girl.
'She does what her sister tells her most of the time, Krista is the bossy one. I call her my little bully.'