This paper presents a multi-input piezoelectric (PE) energy harvesting circuit, which integrates a Series Synchronized Switch Harvesting on Inductor (S-SSHI) and a Voltage Doubler (VD). The proposed circuit uses the VD topology to reduce the number of the diode in the resonant loop of the S-SSHI circuit, and hence more power available for the load. Besides, the circuit adopts a parallel connection for multiple different PE transducers, in which an inductor and VD can be shared to harvest the energy. It improves the utilization of the inductor and reduces the complexity of the circuit. The circuit is self-powered and capable of starting even if the battery is drained. The circuit was prototyped with discrete components with a cost-effective implementation. The measured results show that compared with full-bridge topology, the proposed circuit increases the power harvested from two different PE transducer by 3 times at the same excitation condition.