That's one of the reasons why corporations structure themselves vertically like you just described: they hope to dilute responsibility and compartmentalize legal impacts.
Brian Thompson and Andrew Witty didn't run their bits of the company in total separation: they talked to each other. They are equally guilty, the structure of the company doesn't split the blame in two parts. In fact, you could easily argue that they probably conspired to achieve the same outcome, which makes it even worse.