Complex sentence: He went to work for Bates until he experienced creative differences, and with Ralph Holden, who was in charge of new accounts at Bates, founded the advertising agency "Calkins and Holden".
Split into simpler sentences: He went to work for Bates until he experienced creative differences. He joined with Ralph Holden, who was in charge of new accounts at Bates, to found the advertising agency "Calkins and Holden".