Large and irregular particle sizes of B4C in the Mg matrix are performance-impeding challenges of the as-cast Mg-B4C composites. In an attempt to overcome this, the secondary solid-state friction stir processing of the as-cast Mg-10%B4C composite with a flow-enhancing double-pin tool was carried out and the ensuing result was compared with that of a single-pin tool. The microstructure, hardness, tensile strength, wear, and the fractured surface of the processed composites were investigated and compared. The extra pin-shearing effect and the complex pin-induced interactive material flow of the double-pin tool induce better refinement of the B4C particles in the Mg-10%B4C composite. The use of a double-pin tool increases the stirred and recrystallized vortex/swirl width, kernel average misorientation (KAM) fraction, dislocation density, hardness value at the stirred center (117 HV), and tensile strength (194 MPa) of the Mg-10%B4C composite as compared to the single-pin tool. The double-pin tool changes the fracture path of the composite away from the stirred center owing to the improved material flow and properties of the stirred center. The tribological properties (weight loss, wear rate, and coefficient of friction) of the processed composites are equally improved by the double-pin tool. A double-pin tool is thus recommended for the improvement of material flow, particle-disintegration, mechanical and tribological properties of Mg-based metal matrix composite.