It seems like when opening the Modern UI version of IE from the Start screen, it will only open on the main display. Afterwards, I don't know how to move it to the second display, since Win+← and Win+→ don't work for Modern UI apps. Is this possible or do I need to go to desktop mode and do it that way?
You can move apps between and around your screens.
When you press the Windows logo key + Left Arrow or Windows logo key + Right Arrow, you move apps around your screen. When you press the Windows logo key + Shift + Left Arrow or Windows logo key + Shift + Right Arrow, you move the app from screen to screen.
If you're using a mouse, you can also grab the title bar and move the app where you want it. To see the title bar on a Windows Store app, move your mouse to the top of the app.
I was still having problems getting metro apps to the second screen after reading this post. When I checked the display resolution of my second screen I realised I was not using the "recommended" settings at 1024x768 but a "higher" resolution at 1280x720.
Once I switched my second monitor to the recommended resolution the metro apps would then correctly move over to that screen using the Windows logo+shift+left/right arrow keys as suggested by other posters.
Hope this helps someone