That’s interesting! I wonder if it’s exactly the same whether the twins are identical or fraternal? Same sex or different sex? My niece had twin fraternal girls. One of the twins, from the time she was a very teeny baby, when you’d hold her on your shoulder and pat her back as if to burp or comfort her, she would take her little hand and pat your back in the same way.
HA...my Mother told me I did that when I was a baby....and I was NOT a twin....