(CNN) When Michele Mordkoff and Allison Kanter were 5 months old, they were each adopted by loving families in the New York City area, two days apart. The infants, of course, didn't know it -- and their families weren't informed -- but the girls were twins. It was mid-October 1964, and the two wouldn't see each other again for more than 50 years.

Today, splitting up biological siblings is almost universally discouraged, but at the time, some researchers believed that it was in the best interest of twins to be separated during the adoption process. The thinking was that they wouldn't have to compete for the attention of their new families and would thrive better on their own.

Louise Wise Services, the agency through which Mordkoff and Kanter were adopted, operated by this sibling separation maxim. The women say their adoptive parents were never informed by the now-defunct agency that their daughters had a twin.

"They are just as shocked as we are, if not more," Mordkoff said, "because they felt they were getting full disclosure and information about who we were and what our identity was."

Michele Mordkoff, left, and Allison Kanter around 2½ years old.

Mordkoff had always wondered about her identity, and her curiosity finally got the best of her last summer after she watched CNN Films' Three Identical Strangers ." It's about a set of triplets, also adopted through Louise Wise Services around the same time as Mordkoff and Kanter, who only came to find out about each other after one was mistaken for another at a New York college in 1980.

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