As far as I know, you can't, if your PC can handle having 2 up at once, you could run the main screen at say 1280x800 or something (cropping the top or bottom of the screen) and run the 2nd one at 800x600 (low as possible to avoid too much loss in framerate) and just place it so that you can see the map above or below your main screen

A mod that made the toon /w you or talk in party when something pops up on it's radar would be cool though