All the literature I have read on this system states that the lead unit contained the engine that generated electric power that was transferred to the rear unit via a cable to power the tracks. The driver at the rear had to keep close to the front unit so that the cable did not part and leave the rear unit without the power.
However, the film clearly shows the rear unit manoeuvring independently without the power cable connected.
The rear St. Chamond unit didn't have any power source aside from the cable from the front unit. When travelling on roads the rear unit was connected to the front tractor unit with a tow bar - this was about 4m long. The electrical cable connecting the two units was quite long - about 40+m maximum. (I'd have to go digging in the manual to find the exact length). Excess cable was normally stored in a reel on the front left of the trailer unit. In the videos I've seen of the St. Chamond SPGs there seems to have been one guy whose job was to ensure the cable didn't get tangled. Given the slow speed of the units this wasn't an onerous job.
There are Pathe videos of the St. Chamond units being deployed before the Fall of France in WW2 - just to confuse everyone and/or have maximum propaganda effect the gun vehicle is being driven backwards so the gun faces the front.
