Firstly you cannot use Cubase across dual monitors if you have the “maximize” icon [top right corner] set on your master screen. This will automatically set/fix the Cubase program to show in one monitor only.
You have to click on the double screen icons [ also top left] of your master screen, set to " double down" . This then allows you to drag the Cubase screen on the master monitor across to the second dual monitor.
Its important to identify which is your master monitor, on the left monitor or right monitor. Once you have Cubase now across both screens you can position the mixer in one of the screens, the project in the other, etc. but still must be within the Cubase program screen. Only a few items like the transport bar can go outside of the Cubase main screen.
I have had no problems with dual monitors since the early Cubase days.
I have set up separate workspaces for Recording, Tracking and Mixing. The Recording & Tracking set-up has the Project window on monitor 1 and the MixConsole on monitor 2. Tracking has the Project across half the second console as well. The Mixing workspace minimizes the Project window and maximizes the MixConsole console over both windows.
I hope this helps.
Cubase 7.5 32/64 bit Windows 7 64bit Intel Core i7-3.6Ghz 32GB RAM Steinberg MR 816csx, Mackie MCU