Bio-magnesium (Mg) alloys exhibit excellent biocompatibility and biodegradability, making them highly promising for implant applications. However, their limited strength-ductility balance remains a critical challenge restricting widespread use. In this study, ultra-fine-grained and homogeneous Mg alloys were fabricated using double-sided friction stir processing (DS-FSP) with liquid CO₂ rapid cooling, leading to a significant enhancement in the strength-ductility synergy of the stirred zone. The results demonstrate that DS-FSP samples exhibit simultaneous improvements in ultimate tensile strength (UTS) and elongation, reaching 334.1 ± 15 MPa and 28.2 ± 7.3%, respectively. Compared to the non-uniform fine-grained microstructure obtained through single-sided friction stir processing, DS-FSP generates a uniform ultra-fine-grained structure, fundamentally altering the fracture behavior and mechanisms of Mg alloys. The DS-FSP samples exhibit irregular fracture patterns due to variations in basal slip system activation among different grains. In contrast, single-sided friction stir processing samples, characterized by a fine-grained yet heterogeneous microstructure, display flat shear fractures dominated by high-density dislocation initiation induced by twin formation, with fracture propagation dictated by the non-uniform texture. By achieving an ultra-fine grain size and homogeneous texture, DS-FSP effectively modifies the fracture mechanisms, thereby enhancing the strength-ductility balance of bio-magnesium alloys.