How to tell Prism which display to start on.

Windows

Prism Windows fills one window (with other windows inside of that one, if you open several projects). You can move that Windows around, and move it to a different display. 

Prism 6 remembers where the window is and will restart in the same location on the same display (when possible). You can start two instances of Prism 6, and place each instance on a different display.

If the Prism 5 window is on the primary display, then Prism remembers its size and location. Next time you start Prism, it will appear in exactly the same location. 

If you move Prism 5 to a different display, it gets confused. Next time you start Prism, it will start on the primary display. But you can trick Prism to get it to start on a different display. The first thing to do is click the Restore button near the upper right corner, so the Prism window does not entirely fill the display. Now grab its corner and stretch it, so a tiny part (just one pixel is enough) is on the primary display, while most of the window remains on the secondary display. Now when you quit Prism, it will remember its location and start up next time in exactly the same location with the same size. The trick is that part of the Prism window needs to be on the primary display -- but it can be a very tiny, almost invisible, part. 

Mac

Each Prism document is in its own Window, which you can move anywhere. You can have one project on one display and another project on another. 

When Prism starts up, the first project is always placed on the primary display. You cannot tell Prism to always start on a different display. This is true for both Prism 5 and 6.