From your video, it sounds like your motors are stalling during guiding. What are your current targets for the motors during slew and guide? With those TB6600, do you even have that selection?
As far as the electrics, find an electrical guru to help you out. This is not your forte and taking advice and input from so many people is going to get you into trouble if you do not interpret their information correctly.
I am running 24V into my MaxESP without a problem and I do not have the 12V regulator populated. Since Guy is using external drivers, I don't see any need to run the MaxESP higher than 12V. On the other hand, if you are running 24V to the drivers, you can do the same for the MaxESP, to save one supply. The regulator supplied in the current schematic for the MaxESP is the 580-OKI78SR5/1.5W36C which is rated for 36V max. If you use 24V for you limit you will be fine. If you need more voltage to the drivers, then go back to two supplies.
Regarding adding more wires mucks things up, well that is a broad statement without context. The grounds must be there, and they must be low impedance. If it needs a wire to make it happen, it needs a wire.
My suggestion if you have some distance from the supply to the driver, is to add a large capacitor to the power terminals of the driver (50uF, 50V). There may be some interaction or resonance on the wires between the two drivers. Try this: while you are getting the screaming, unplug one of the drivers and see what happens. Please provide the distances between the power supplies, the MaxESP, and the drivers.