Hello Antolne and Thanks for your response!
there is only one method to add multiple material to the same obj in rhino. its not a obvious feature i think it was added in rhino 6 after many user asking for it for a long time its not very robust but it gets the job done and it works only on nurbs surfaces and not meshes.
it works by first you selecting a face of obj like a cube in sub-Object mode (selecting while pressing Ctrl+Shift) then in material panel select a material and right click on it and select “Assign to Objects” for this to work you need to first assign a material to the whole obj and add the sub-obj materials after it. if you dont do this it would still works but rhino will automatically create and add a white material to your obj.
in the sample i first add the red material to the cube then select one of the faces and added the blue material
Sub-Object Material Test.3dm (60.4 KB)