I have done the combo. I usually apply Zaino and enjoy the fresh coats for a few weeks or about a month. Then I will add the Souveran. Basically, I add the souveran for the looks over Zaino. The Zaino underneath gives me the durability and the souveran adds to the looks. Obviously if you add the souveran over Zaino, you wont be able to add more Zaino until you remove the souveran.
I have used Souveran and P21S. IMO P21S is a little brighter and has **slightly** better durability than souveran. Souveran is more wet and warm imo. Some people do not justify the price difference. Put it this way, when I run out of my Souveran I will buy another can, Im about to run out of P21S, but I dont see myself ordering anymore.