Identical twins Grace and Virginia Kennedy were born in Georgia in 1970.
Because they weren’t socialized properly as young children, they ended up inventing their own twin-language (an idioglossia). They even created new names for themselves. They were Gracie and Ginny to everyone else, but Poto and Cabengo to one another.
In the documentary Poto and Cabengo (1980), you can hear the twins speaking their language [vid]. I think you can even hear one call the other “Cabengo” a couple of times [2:31 & 2:49].
How did they come up with the names “Poto” and “Cabengo”? We’ll never know. The twins themselves may not even remember. (I wonder what they call each other nowadays…)