The twin turbo-shaft engine according to the invention comprises a high pressure rotor and a low pressure rotor, at least one accessory gearbox driven by mechanical transmission means, main drive pinion means, on each rotor, driving power transmission means each associated with an input to a differential, the output of which drives a drive shaft of the accessory gearbox. It is characterised by the fact that the power transmission means extend in two different structural arms, the power transmission means of one of the rotors being connected to a power transfer box fulfilling two functions, firstly transmission of power to an input to the differential, and secondly modification of the transmission speed.The required space in each arm is thus reduced and the efficiency of the accessory gearbox is high.