I'm playing 2 on a Mac and I'd love to have the game running full screen on a secondary monitor. Currently the game is launching to full screen on my Macbook and killing the external monitor. Opening a windowed version launches to my Macbook again, and it won't let me drag the window around. I'm currently playing with my display settings on mirror but it feels odd to be covering over my Macbook display when I could be using it to look up spells etc from the game manual.

Any help, much appreciated.

I'm not a pro on this on a Mac, but "Mirror" doesn't let you drag the window over, because the other monitor is just a mirror. To do so, you usually have to extend your desktop to both displays, to get one area split upon two displays. (Command-F1 should toggle between the two modes)

Check these links to Apple's How-to articles:
http://support.apple.com/en-us/HT5019
http://support.apple.com/kb/PH13814
http://support.apple.com/en-us/HT5891