Conductivity is a material property of the body, not a boundary condition. It is the external faces of the body that can have a boundary condition. If no boundary condition is applied to an external face, it is automatically an insulated face (heat flow = 0). If you use shared topology between solids, after meshing, the coincident faces between solids is no longer considered an external face. Each body has its own conductivity and heat flows across the coincident shared internal faces. If two bodies have a contact defined between them instead of shared topology, the contact by default will have perfect thermal conductivity (zero resistivity), but you can optionally add some thermal resistivity to heat flow at the contact surface.