Complex sentence: They had two sons, Julian and Quentin, early in their marriage, but by the First World War both Vanessa and Clive had other sexual partners, most with the full knowledge of one another, but they never divorced.
Split into simpler sentences: They had two sons, Julian and Quentin, early in their marriage. By the First World War both Vanessa and Clive had other sexual partners, most with the full knowledge of one another, but they never divorced.