What that means, we figured out, is that the twins probably "think" about each other as part of themselves. There is not this "other baby" in here with me. There is "me", and sometimes I kick myelf in the head or I take up too much space and squash myself and sometimes, I can cuddle with myself.
But it certainly explains why twins should share the same bassinet in the hospital and why they sleep better beside each when they are still newborns/infants. Otherwise, they are missing half of themselves.