I mentioned it because good friends of mine growing up were Scott & Greg Sullivan, identical twins who didn't speak English until they were 4 but developed an apparently extensive and coherent language with each other.
When I knew them they'd forgotten their private language, and couldn't tell me anything about it. I heard the story from their mother.
Ever since then I've wondered how elaborate, how complete such languages are. Whether there's any similarities from twin-pair to twin-pair. Whether there's any grammar.
Steven Pinker would claim this indicates a 'language gene' with certain built-in grammar and syntax -- but from what I've read, the answers to all of my questions are "no" so Pinker would be wrong on this.
Still, an interesting study.
I know people thrown together by unusual circumstances, such as slaves from different regions or workers working overseas -- if they share no common language, tend to develop pidgins with basic reverse word-order grammar (like Yoda) and words based on common Indo-European roots. But of course newborn twins wouldn't have that background (although they would hear language around them).
Weird stories abound.