The two-layer sloshing of water and diesel oil is studied numerically by using the consistent particle method (CPM). CPM is a projection-based particle method with the distinct feature of having consistency and high-order accuracy in computing spatial derivatives. A validation against published experimental data is conducted. Then, the two-layer sloshing under swayonly and coupled sway-heave excitations is studied. For the coupled excitation cases, it is found that the sum/difference of sway and heave frequencies being close to the odd multiple of the system’s natural frequency induces secondary violent sloshing even when both excitations are further away from the natural frequency.