Krysta and Tatiana are conjoined twins attached at the head. Conjoined twins are very rare, but their situation is even more unique than most. The girls share neural tissue in their thalamus, which they share.
They are able to see through each other’s eyes and feel what the other one feels. When you tickle one sister the other one smiles and laughs. When an object is out of the line of sight, but not the other, the first girl can still reach for it and know where it is.
Most interestingly, though, is that they believe the girls have the ability to share thoughts or hear each others’ thoughts! Despite this, they have very separate personalities. Tatiana is laid back and Crysta is a bit of a bully.
Their parents knew at 21 weeks of pregnancy that the girls were conjoined at the head and never doubted that they wanted to see the pregnancy through to the end. The girls are completely mobile and run around together. However, it is impossible to separate them, though, due to the connecting brain tissue that they share.