Use the Viscoelasticity subnode to add viscous stress contributions to a
Layered Linear Elastic Material or to a
Layered Hyperelastic Material.
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 Layered Linear Elastic Material or
Layered Hyperelastic Material node selected in the model tree: