Unfortunately your image shows you don’t have the mixer set to fullscreen outside the Cubase main window. We are aware of the fact the traditional method by keeping the mixer within the main frame still works.
What we want is the mixer full screen to monitor X and the main window maximized on the other monitor.
On OSX there’s no problem, you can use the fullscreen mixer how Steinberg intended it. So in other words, you don’t have to stretch the Cubase main window across both screens when you don’t want to.