As you are probably aware, there is no setting in FCP to choose which display to use as the second display.
Also, there is no way in System Preferences to designate the second display; however, there is a simple way to designate the primary display, and that is what we use to achieve the desired result.
This involves a little trip to System Preferences->Display.
Here I have 3 displays. The middle one has the whitish menubarlet on it; this designates it as the primary display:

Now here is what you do:
1) Drag the whitish menubarlet to the display that you want to act as the secondary display in FCP.
2) Start FCP in the display where you want its primary interface to be (*)
3) Window->Show in Secondary->Display->....
Done
(*) You may wonder about this one:
There are several ways to do this:
2a) Start FCP and drag its window, if necessary, to the desired display
2b) Click the menu bar of the appropriate display, to make it the active display; and start FCP using Spotlight from the keyboard to ensure it starts already in that display
2c) If you have more than one desktop in at least one display, you can force FCP to always use the same display: click and hold the Dock icon, choose Options->Assign to->Desktop on Display n (you may have to figure out how the displays are numbered, but after that it should stick)