Complex sentence: One day after the editing was done, Wasserman invited Landis up to his office to speak with Ted Mann, head of the Mann Theatres chain, which dominated film exhibition in the Western United States.
Split into simpler sentences: One day after the editing was done, Wasserman invited Landis up to his office. There with the executive was Ted Mann, head of the Mann Theatres chain, which dominated film exhibition in the Western United States.