Thanks all, again, for the so constructive comments!

Martin... it was not easy to have good results at first for me too... I had to pass many hours studying carefully Chipp Walter's -and other peoples' too- tutorials in Youtube. : - )))

Michael... thanks a lot for your so instructive comments. The hint about joining the two halves -by having empty the faces to be 'glued' and using the 'join' command- works. I tried it and it is ok. Some such hints are extremely important for understanding the way the app works.