The magnetohydrodynamic (MHD) instability of the liquid-liquid interface inside aluminium reduction cells is studied computationally. In aluminium reduction cells, conditions of the cell cavity, anode, and cathode assembly influence the distribution of electric current, and subsequently, the Lorentz force resulting in waves at the interface of molten aluminium and bath. The interface with pre-existing disturbances gets further perturbed by the scheduled operational activities in the aluminium smelter. Therefore, to study a practical scenario, a bichromatic perturbation with different wavelengths and independent wavenumbers along the cell length and width is considered here for the study. A 3D MHD model based on PHOENICS-ESTER is used to study the effect of cell length-to-width ratio. It is observed that, after an initial bichromatic perturbation, the amplitude of the interface oscillation increases with the increasing cell aspect ratio. However, in the case of a monochromatic perturbation, which is considered in most of the reported works, we see an opposite trend.