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I think what might have happened is that the mounting plate acted like a leaf, spinning quickly around but dropping relatively slowly to the ground.
As the two cameras most certainly have a different weight but not that much different, the system is not in balance and hence one side of the plate would turn down and that would put the whole system spinning. Also, the plate would absorb most of the force when hitting ground.
I don't think the relative mass makes any difference at all. Gravity acts equally on all bodies irrespective of mass which is why a brick and a penny dropped from the same height will hit the ground simultaneously.
Of course they will drop at the same speed - after all, they were attached together by the mounting plate.
However, the mounting plate would not stay horizontal if the weights at its ends are not the same. There is force of gravity affecting to the plate and to each camera. The force depends on the weight of the object, so a heavier object is subjected to a larger force. Though, getting a heavier object moving does require a proportionally larger force, so if the cameras were dropping individually (and in vacuum), there would be no difference in their speed.
But in this case the cameras were attached together, on the mounting plate, and there is air. The air causes a force that resists the falling. This force is acting on the underside of the mounting plate, equally everywhere. However, the heavier camera is pulled more by the gravity, so the plate tilts towards that side.
Now comes the part where only speculation is available. The mounting plate will most certainly begin rotating, but does it rotate horizontally or vertically or both, regularily or irregularily? How much does that rotation affect it aerodynamically? Anyone willing to borrow two cameras and a mounting plate for some tests?