Collective cell migration plays an important role in maintaining the cohesion of epithelial cell layers, in wound healing, and in the gastrulation phase of embryonic development. A recently developed continuum mechanical model of the migration of a single cell layer based on an assumption of elastic deformation of the cell layer leads to a generalized Stefan problem. Analysis and numerical results indicate that a large class of constitutive equations for the dependence of proliferation on stretch leads to traveling wave solutions with constant wave speed. The model is extended to describe the migration of two coupled cell layers, and a numerical method based on a level set method for free boundary problems with domain decomposition is constructed.