Abstract: To address the issue that existing multipath routing research in underwater acoustic networks (UASNs) is largely confined to the only network-layer analysis of spatial topological characteristics, this paper proposes a cross-layer analytical model for multipath redundancy performance trade-off. This model aims to analytically dissect the trade-off between the concurrent interference cost and the spatial diversity gain induced by multipath redundant transmission. By introducing the "spatial decoupling factor" and the "inter-path collision factor," the proposed model quantitatively characterizes the spatial coupling features of multipath routing. Combined with the Poisson distribution characteristics of UASN traffic, the dual-path spatial coupling characteristics are mapped to the concurrent traffic increment of the lower channel, and the closed-form analytical expression for the single-hop transmission success rate under contention-based MAC protocols is derived. Furthermore, a joint expectation equation for the end-to-end packet delivery ratio (PDR) of multi-hop dual-path redundant transmission is formulated. Theoretical analysis and simulation results demonstrate that in low-load scenarios, when the spatial diversity gain acquired from inter-path spatial decoupling significantly outweighs the concurrent interference cost of the dual-path mechanism, the end-to-end delivery ratio of multi-hop transmission is substantially improved compared to single-path transmission.