Complex sentence: The Prince of Conti went to Poland to inspect his possible new kingdom, while Marie Thérèse stayed in France, and during this time, the "princesse de Conti", became the Titular Queen of Poland and her husband the king.
Split into simpler sentences: The Prince of Conti went to Poland to inspect his possible new kingdom, while Marie Thérèse stayed in France. During this time, the "princesse de Conti" was the titular Queen of Poland and her husband the king.