This is one of Kathleen Gilles Seidel's early works, but it's just as good as her later novels, IMHO. I didn't love it the first time I read it, but it stuck with me since the plot is unusual. I've re-read it since then and appreciated it even more. (BTW, the book I have listed is my second copy. One copy will stay on my "keeper" shelf.)
They could never forget they were mirrors ...and mistakes
They were very proper Bostonians who worked hard, dressed conservatively, and ate and drank in moderation. Suzanne Lawrence, secretary to the vice-president of Southard-Colt, and Patrick Britten, the firm's most brilliant consultant, hid behind polite, cool facades--and led lives of exquisite loneliness. Identical in taste, temperament and habit, they drew together in the belief that they would always remain friends. For having known nothing more, they expected nothing more.
But they never anticipated the powerful instinct that would well up in Patrick. It would teach them about love--and despair--and would bind them together forever.