The goal of energy harvesting field is to produce electric power from ambient broadband forcing. While there have been various proposals to use nonlinear effects to broaden the frequency range of energy absorption, it has not yet been clarified how to design the nonlinearity for the most efficient operation. This paper shows a possibility of energy harvesting from a multi-frequency force using a system of coupled Duffing oscillators. Using this example, we perform detailed numerical studies to illustrate that the energy absorption increases when the harvester’s velocity becomes maximum during phase synchronization between the two oscillators. We also compare the energy absorption between that system and a resonant mass-spring system. We show that for the devices base on coupled resonators, the performance can be optimized by choosing a system with the optimal path in the phase difference/velocity plane. This criteria opens a possibility to design the nonlinear characteristics of the energy harvesting devices.