heavy cavalry engaging heavy cavalry

[Urselius]

The three horsemen are stunt-riders on highly trained horses, the average cavalryman and mount of the past were probably less adept.

Quite the contrary - they were more adept, in case of elite heavy cavalry formations at least.

I think a squadron of 100+ men in two ranks galloping at speed, trying to keep knee to knee over imperfect ground would quickly degenerate - with many falls and also injuries to riders legs from banging against a neighbouring horse's flanks.

I've read that there is a pre-WW2 newsreel showing a Polish cavalry parade, during which a unit of cavalry was galloping at speed in a close knee-to-knee formation (without falls and injuries!).

The stunt rider has to control his horse and remember to look good for the camera. Not that much to do, and they can do these things very well. A cavalryman in action has much more to do, obviously if he were well trained he would fight better than a modern stuntman, who does not have to fight - just appear to do so occasionally. Also a stunt is very carefully planned and all methods of making it safe are employed - a cavalry charge is primarily intended to damage the enemy - looking over the ground beforehand, or choosing the ground to be charged over would usually not take place to any great degree.

Imagine a squadron of two ranks, knee to knee galloping at the charge. One horse in the front rank hits a pothole or is felled by a missile, it goes down, it may take out one or both of the horses on either side, and also one or more horses in the following rank - as they cannot swerve aside because they are hemmed in. Not an efficient way of making a charge - charge knee to knee at the trot, or charge at the gallop in a more open order.

Galloping over prepared ground in a parade is, again, not the same as over uncertain ground on a battlefield.