I would like to add my two cents...
I have a twin sister. We are a special type of twins that many people argue don't exist. We are half identical twins. Meaning we are identical twins
(other than the fact I am 1 inch taller) but born from separate "sacs".
Growing up I remember me and my sister having conversations like these. My mother says my sister and I would spend hours just "baby babbling" to
each other.
Eventually we made up our own language consisting of facial expressions, body expression and vocal sounds.
Our older brother was never able to figure it out