Use the Viscoelasticity subnode to add viscous stress contributions to a
Linear Elastic Material, Layered or to a
Hyperelastic Material, Layered.
If the same layer is selected in two Viscoelasticity nodes being active on the boundary, then there will be two viscoelastic contributions.
Physics tab with Linear Elastic Material, Layered or
Hyperelastic Material, Layered node selected in the model tree: