Dual row retaining walls can form efficient port and embankment structures, or even be used as coastal defence against Tsunamis. The system of parallel sheet pile walls can have a large lateral capacity within serviceability limit states due to the combined strength and stiffness of the walls and confined soil. Optimising the design by reducing the wall section and ensuring greater utilisation of the soil capacity has economic and environmental benefits but requires a deeper understanding of the dynamic soil-structure interaction. Centrifuge and numerical modelling is used to elucidate the mechanics of two systems with relatively flexible and stiff walls. Considering the fraction of the walls plastic moment capacity mobilised alongside the peak deflections illustrates the advantages of using relatively flexible retaining walls in these systems. More fundamentally, the importance of vertical variations of both the stress and strain during the horizontal dynamic loading is shown. The limiting horizontal stresses and phasing of the stress components around the walls are better understood by considering the mobilisation of earth pressure coefficients, reinforcing previous work which recommends a move away from conventionally defined limiting dynamic earth pressure coefficients.