I once worked with a waiter who had twins, and they had this. They'd talk together in gibberish, yet act as if they perfectly understood what the other one was saying. I found it fascinating.
So I was rather pleased to discover there was a word for it.
Link: https://en.wikipedia.org/wiki/Cryptophasia
"Cryptophasia is the phenomenon of a language developed by twins (identical or fraternal) that only the two children can understand."