Implementation of Dirichlet-Neumann method

There is a package called FEMAddons that provides utilities that may be of help when implementing domain decomposition methods, and there is an article using it here.

AFAIK, Neumann-Neumann and Dirichlet-Neumann methods cannot be implemented because to implement them you need to be able to enforce boundary conditions such as

$$ partial_{n_1}u_1=partial_{n_2}u_2, $$

where $n_i$, to paraphrase Wikipedia, is the unit normal vector to the boundary shared by the subdomains $u_1$ and $u_2$. If you look at NeumannValue, you will see that it allows you to specify boundary conditions of the type

$$ overset{rightharpoonup}{n}cdot (c nabla u - gamma + alpha u) = g - q u, $$

where the constants are defined implicitly by the PDE. However, from what I understand, enforcing, in general, the Neumann-Neumann boundary condition stated above is not possible.

(Always happy to be corrected though.)