Existing studies concerning the superposition characteristics of underwater explosion shock waves are mostly limited to the near-field range， while investigations on shock wave behaviors in the mid-near field remain insufficient. In this paper， the Arbitrary Lagrangian-Eulerian （ALE） method is adopted to conduct numerical simulations on the shock wave loading characteristics under the simultaneous detonation of two explosive charges. The reliability and computational accuracy of the numerical model are validated by comparing the simulated results of free-field single and dual-charge underwater explosions with theoretical solutions and experimental data. Through parametric analysis with variable charge separation distances， the influence law of charge spacing on shock wave loading characteristics is systematically explored. The results show that the shock fronts synchronously propagate and arrive at the symmetry plane of the two charges， inducing a prominent shock wave superposition effect. When the normalized spacing h/l exceeds 0.4， the superimposed peak pressure is higher than that generated by a single charge with an equivalent total mass， whereas an opposite superposition rule occurs when h/l ≤ 0.4. The maximum relative difference in peak pressure reaches 71.23% at a normalized spacing of h/l = 2. On the asymmetric plane， shock waves from the two explosion sources arrive asynchronously， forming a typical double-peak pressure curve. The first pressure peak is solely generated by the near charge， while the second peak， though similar to the pressure characteristic of the individual far charge， presents a pressure amplitude increase of 3.19%-13.41% and an advanced arrival time. Furthermore， for a fixed total charge mass， an optimal charge spacing exists that enables the peak pressure amplification on the symmetry plane and the secondary peak pressure enhancement on the asymmetric plane to be remarkably superior to those under other spacing conditions.