Meshing with different materials

[gabburgio]

Hello everybody

I have a question for the people meshing for moltres in gmsh.
How do you go about defining multiple material zones in gmsh?
I have of course used physical e.g. surfaces, but I end up with them being disconnected meshes rather than subzones of a single mesh.

To make this clearer I have a minimal example, where I want to mesh a region composed of two rectangles, one right on top of the other.
If I create first one rectangle, then the other, then I make each one a physical group and mesh, my solution ends up being discontinuous across the central boundary, since it is actually being carried out on each subdomain separately (and material properties differ on the two domains):

I tried clicking on "unpartition" after meshing to no avail. Is there any other way I am supposed to use unpartition to make the various meshes part of a single mesh?
Deleting the line between the two rectangle before meshing but after having defined the physical groups also accomplishes nothing.

I have also tried compund lines/surfaces, which do produce a single mesh, but one in which some elements cross material zones, which produces an error with libmesh because they have several subdomain IDs, and also doesn't really make sense because of the discontinuity inside the elements.

So how am I supposed to do this, in a way that is simple enough to be practical on more complex geometries?

Thanks in advance