Savannah Purchase.
3rd book in the Purchas/Purchis saga. Can be read alone & is the best of the saga. As children, Juliet and Josephine often played the innocent game of impersonation. They were cousins, but they looked enough alike to be twins. Life and war separated them, but the years didn't dim the astonishing resemblance. Now Fate suddenly threw them together again -- two beautiful, desirable women playing out a deadly masquerade