Complex sentence: He engaged in business with Edward Livingston, who was then district attorney and mayor of New York, and, after moving to New Orleans, formed a professional partnership with his brother-in-law, Beverley Robinson.
Split into simpler sentences: He engaged in business with Edward Livingston, who was then district attorney and mayor of New York. After Livingston moved to New Orleans, Duer formed a professional partnership with his brother-in-law, Beverley Robinson.