Move secondary display to different monitor

I am using my Macbook with three external monitors (dual editing display plus A/V output) and would like to use the screen of the macbook as well for editing the text of the film in a word document.  
Now If I open my macbook pro the secondary editing display (for my media browser window) moves to its display although my main display is still set to my external monitor in macos display settings.
I cannot tell the display to move to another monitor,  if I select the secondary display window and select in the menu window->move to->(external monitor name) it only moves my FCPx primary (main) window. How can I configure the secondary display to be shown on an external display as well and use my macbook display to edit the text of my film in a word document??