Owing to the effectiveness in addressing the challenges posed by the uncertainty and instantaneous volatility of renewable energy, the complementary exploitation of diverse energy sources has received considerable attention from researchers worldwide. In this context, this study proposes a two-degree-of-freedom marine energy harvesting system to concurrently extract energy from tidal currents and offshore winds. The proposed energy-harvesting scheme consists of two mechanically linked oscillators with alternative bluff bodies, capable of scavenging multiple forms of fluid energy based on flow-induced vibrations. An associated solution method for the fluid-structure interaction was numerically developed to investigate the dynamical response and energy-harnessing performance of the proposed system in detail. The simulation results demonstrated two typical working modes that occur under various inflow conditions. The 1st mode is used for energy harvesting, whereas the 2nd mode is used for vibration alleviation. These modes can be used to promote the converted power and vibration reliability of the proposed system. In contrast to the conventional one-degree-of-freedom energy harvester, the proposed system demonstrated an improved converted power, increasing from 39.13 W to 44.18 W, and a reduced maximum displacement, decreasing from 1.98 m to 1.02 m, exhibiting superior comprehensive performance in terms of converted power, energy conversion efficiency, adaptability to inflow velocity, vibration reliability, and power mix flexibility. Hence, the proposed system offers a novel perspective for the development and implementation of marine energy-harvesting technologies.