There was a video that was funny it shows the 18 month old twins talking and they were really talking back and forth. It seems they understood themselves very well.
I have a cousin who are twins. They are girls but they are not identical. For their parents, i guess they are thankful that it happened that way because it looks like you gave birth at two different times. They are close the bond is really there. I believe twins have their own special bond from the very beginning of their journey in their mother's womb until they were born. Bet they were already communicating inside the womb.
I love to see families with twins and triplets but more than that scary! Needs people to help and should be financially secure as it is expensive to raise babies. I love big family but financially unstable.