To me this was impossible to achieve in 3d-Coat alone until recently with the addition of a checkbox in the beta tools section of preferences : "Treat retopo groups as objects", which is ticked by default, and needs to be unticked for the following to work. Unchecking "Treat materials as separate textures" simply had no effect, whatever the initial conditions of materials. (If you know what it did and how to make it work, please let me known since the beta tools will eventually be disabled in