Another tragic, but avoidable accident.
As soon as I saw the video of the two aircraft apparently flying parallel to each other it brought back memories of my earlier times being required to fly in military “box” formation at low level. It can go badly wrong very quickly and I would never do it out of choice. I was once leading one such formation where our number two suddenly appeared from our left, very close and flying at us from our half past nine /ten o’clock position, which was across the cockpit. He had lost sight of us and without saying anything tried to rejoin by cutting a corner. I saw him first and had to go hard left to pass behind him!
Loose “Arrow” formation is far safer.