A method for starting an aeronautical free turbine and single-spool gas generator turbomachine of a twin-engine aircraft including two independent electrical networks each comprising a 28V battery selectively powering a starter-generator, wherein to ensure, under the control of a turbomachine regulation computer, the startup of the gas generator turbomachine first at a nominal voltage of 28V by putting the two batteries in parallel and then at a nominal voltage of 56V by putting the two batteries in series while avoiding too rapid acceleration of the gas generator turbomachine, the serialization of the two batteries is ordered only once a combustion chamber of the gas generator turbomachine is ignited and the speed of the gas generator turbomachine is greater than a predetermined speed threshold N1 for ensuring, through this serialization, a positive acceleration margin at a point of maximum drag of the gas generator.