Mechta
Yes the Piasecki incident represents one failure mode: Air Resonance. It is though an example of tying multiple helicopters together. There are so many other failures with rigid connected or independent suspension that will result in the same outcome. Your video shows just how low a pilot's IQ has to be for the technique to be used. What happens when one of a set loses an engine and pickles the load? It is easy, the others are screwed. One note: helicopter hook structures are design for primarily vertical loads, not a 50% lateral load if someone is stupid enough to connect long lines directly to a single load point.
The Sultan