<p>In this paper, the problem of non-uniform and asynchronous filling of two cavities in multi-cavity compound squeeze casting is systematically investigated, and the underlying mechanism responsible for this phenomenon is elucidated. By combining metallographic analysis of the pressure chamber biscuit and the ingates on both sides with numerical simulations of the heat transfer behavior of molten metal in the chamber and the evolution of the pre-solidified shell thickness, the fundamental cause of non-uniform filling is revealed. The results indicate that a cup-shaped, open pre-solidified shell forms in the chamber prior to cavity filling due to intense heat transfer between the molten metal and the chamber wall. During the squeeze process, this pre-solidified shell is pushed upward by the punch and accumulates near the ingates, where it causes significant obstruction to molten metal flow. Under the applied filling pressure, the molten metal preferentially and randomly breaks through one of the side ingates, enabling the filling of the corresponding cavity. However, by the time this breakthrough occurs, a substantial portion of the filling pressure has been dissipated, making it insufficient for the molten metal to simultaneously overcome the obstruction at the opposite ingate. As a result, severe non-uniform filling between the two cavities occurs. At present, no definitive solution exists for this intrinsic non-uniform filling problem in multi-cavity compound squeeze casting. Nevertheless, in practical production, the fluidity of the molten metal can be effectively enhanced and the filling resistance reduced by optimizing process parameters (e.g., increasing the pouring temperature and mold temperature), improving ingate geometry, and optimizing the structural design of the chamber, thereby significantly mitigating non-uniform filling and improving filling performance.</p>