Smithy
Thats ok as didnt think it was personal
i think the more experienced you are as a pilot the more likely you are going to give the controls to a non pilot.
Experience breeds confidence. As you have done it so many times before your happy to sit back and give a non pilot the thrill of their lives always knowing you can take control.
These instructors were experienced and could quite easely have let the non pilots try formation flying with small corrections by themselves. But small corrections wont stop a large unexpected input with disasterous results at such close proximity.
I am not saying that is the scenario but it is a likely situation which could have answered why two experienced pilot would collide in formation.
Whatever sadly someone messed up big time.
Pace