Difficulties and errors when trying to apply Robin Boundary Conditions with data from another domain (Eigenvalue Problem)

variational formulation
2

I would suggest using the MixedDimensional Finite element method implemented by @cdaversin, whole paper can be found at: https://arxiv.org/pdf/1911.01166.pdf

Using a MeshView for each domain should make it possible to transfer data between the two domains.

You could Also consider interpolating between the non-matching meshes, see: Data projection from one mesh to another mesh