Block preconditioner for multigrid

[Chayapol Chaoveeraprasit]

Dear all,

I am trying to use a multigrid preconditioner to solve a PDE with two domains, in which the resulting matrix will be a block matrix. I am trying to replicate what is in step-16 tutorial program, but turns out it doesn't work since there are separate dof handlers for two domains, and I have no way to construct the PreconditionMG object to handle the dofs from both domains (since we can't combine two dof handlers into one). 

Does deal.II provide a block preconditioner for multigrid, or do I have to implement my own from scratch by inheriting from Subscriptor class?

I would greatly appreciate your help in this matter.

Regards,

Chayapol