Elyse Schein had always known she was adopted, but it wasn't until her mid-thirties while living in Paris that she searched for her biological mother. What she found instead was shocking: She had an identical twin sister. What's more, after being separated as infants, she and her sister had been, for a time, part of a secret study on separated twins. Paula Bernstein, a married writer and mother living in New York, also knew she was adopted, but had...