Your other screen may be any screen that your mac recognizes as such. It can be an external monitor, connected to your mac's miniDisplayPort port; it can be an iPad, using software like "AirDisplay"; it can even be another mac, using software like "Screen Recycler". But even in this last version, your other mac is just being used as a display, it NOT running FCP X itself.
The point to make here: you run ONE instance of FCP X in ONE computer; this computer can have TWO monitors, and FCP can use them - to some extent: you can, for example, place the Event Library in one separate monitor, or place the Viewer. You can't move the separate items of the interface (timeline, inspector, ect) at will, you have to use the (limited) options is the application. Although technically the FCP X interface can be seen as comprising several "windows", you can't just move them around independently.