A rotary power source directly drives an epicyclic gear set, or drives the epicyclic gear set through a transmission device, and a respective controllable multiple speed-ratio device is individually installed between two output shafts of the epicyclic gear set and the loads driven thereby, to enable the wheel sets of the driven loads to be differentially driven with a different driving speed ratio and driving torque while propelling a combined common load. A limited slip differential or stabilizer device may be connected between the output ends of the two controllable multiple speed-ratio devices, the limited slip differential or stabilizer providing a slip coupling torque to stabilize driving of the respective individual loads, as necessary.