Double Clutches - Means two clutches. Old trucks had two so that if one failed, they could use the other.
On a serious note - the correct term is double de-clutching. Older trucks/buses with constant mesh gearboxes required that you press and release the clutch pedal twice when changing gears - one to move to neutral and the next to move to the correct gear. With a blip of throttle(accelerator, I mean - there is no throttle in a diesel engine) to match revs when downshifting.