Complex sentence: Whilst at Universal he established relationships with George Roy Hill and Clint Eastwood, which began on "Slaughterhouse-Five" and "High Plains Drifter" respectively, he went on to work with both men on numerous films.
Split into simpler sentences: Whilst at Universal he established relationships with George Roy Hill and Clint Eastwood, which began on "Slaughterhouse-Five" and "High Plains Drifter" respectively. He went on to work with both men on numerous films.