On windows os your mix window and edit window are contained within one overall window.
You have to drag the bottom right corner of that 'main workspace window' so it occupies both screens.
Then you can move the edit/mix windows around within that.
On mac mix and edit windows are completely separate so you just drag each one wherever you want it.
If that doesn't work, just buck it out the window.
(PS, I know I said window too much there.)