Complex sentence: In New York City from 1960 to 1963, he had a relationship with Barbra Streisand, including living together for a year, during which time he helped her develop the nightclub act that began her successful career as a singer and actress.
Split into simpler sentences: In New York City from 1960 to 1963, he had a relationship with Barbra Streisand. They lived together for a year, during which time he helped her develop the nightclub act that began her successful career as a singer and actress.