The Silent Twins, June and Jennifer Gibbons
These two are also known as the Silent Twins. The twins were inseparable from the time they were born but spoke only to each other in a language that nobody else understood. They did not interact with other children and the situation got so bad, that they had to be separated. However, this only made the problem worse and the two got withdrawn and catatonic. After they got reunited, they came to a decision that one of them had to die. Jennifer took the plunge after which June started speaking and communicating with others.