What you're describing is a dilution of responsibility. The first firm didn't do their due diligence, and assumed that Flint was doing something it wasn't. The second firm didn't do their due diligence, and simply overlooked the lead issue. I don't find this to be terribly convincing; it sounds like the both firms should be on the hook, because they both failed. As water engineers, they should be on top of this stuff. That two firms failed doesn't demonstrate that they shouldn't be responsible, it means that the city hired two incompetent firms which individually assumed and then neglected their responsibilities.