In the present work, a cooperative hybrid energy harvester for scavenging ambient vibrations is presented. The proposed energy harvesting system exploits both piezoelectricity and electromagnetism to harvest rotational energy. More precisely, while the electromagnetic device is devoted to energy harvesting, the piezoelectric element is actually used for efficiently converting the energy of the former. Indeed, in order to improve the electromagnetic transducer's AC/DC conversion efficiency, a half-wave voltage doubler where the piezo element is driving MOSFETs is employed. Such a motivation is actually explained by the much lower conversion abilities of the piezoelectric transducer compared to the electromagnetic one in the considered structure. Simulation and experimental validations are presented, demonstrating the improvement of the energy conversion efficiency of the electromagnetic transducer.