Parents respond to their children. It requires understanding spouses. Twins serve an important function for one another. Twins may be very close to their spouse but it will be a different relationship. Some spouses may not fully understand what it means to be a twin.
Some twins marry other twins because they know what it is like to have a twin.
It is a private system of words or sounds or gestures that sometimes develop between twins. This kind of behavior is like what happens between spouses or best friends. When this happens between young twins, it might slow down the development of normal language, but perhaps if you find an older sibling, they can translate for you. In my book, I discuss six pairs, three identical and three fraternal. Isabella Rosselini has a twin sister.