And then I realized that maybe the secret language is right under my nose....just one word with varying intonations!
When they were teeny-tiny , we always said their first word would be "hey" because they made this noise all the time that sounded like the word "hey". Oddly enough, they now say "hey" all of the time. To each other, to us, to the dogs, to their dolls, to strangers on walks.
Here is a short version of the "hey" noise from when they were 10 weeks old.
And here they are now at almost two years old saying "hey" to each other 50 thousand times before they go to sleep.
Coincidence? Or highly advanced multiples language...