Using three-dimensional (3-D) instantaneous dynamic models of the Central Aleutian subduction zone, we show that a low viscosity, low density (δρ = 10 kg/m3) region in the wedge is needed to match observations of topography, the geoid and the stress-state in the slab and overriding plate. Previous models of the Tonga-Kermadec subduction zone also require a low viscosity, low density (δρ = 20 kg/m3) region in the wedge. A low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab, reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down-dip tension to down-dip compression. If the low viscosity region is large, as in the Tonga-Kermadec models, then the decoupling may also cause the overriding plate to go into extension. These are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones.