Complex sentence: Cooper's private practice began in 1979, when he founded Alexander Cooper & Associates in New York City which, since the partnership with his Yale School of Architecture classmate Jaquelin T. Robertson, is now called Cooper, Robertson & Partners.
Split into simpler sentences: Cooper's private practice began in 1979, when he founded Alexander Cooper & Associates in New York City. Since the partnership with his Yale School of Architecture classmate Jaquelin T. Robertson, the firm is now called Cooper, Robertson & Partners.