when you say you can't get it to work, what are the actual symptoms? What should happen on enabling the dual screen mode is that PCB designs and Schematic designs are 'channelled' to 2 separate instances of the application.
On the first initialisation it should attempt to arrange these program instances onto the two monitors, but if it fails to do this then its possible that both instances would appear on the one screen. You can then drag one to the other screen and position it. The positions should then be retained for future invocations.
In fact contrary to what many people think, you don't actually need two monitors to use the dual screen option. Because it simply works as two independently-positioned program instances, they could just as easily be positioned side-by-side on a single widescreen display.
Anyway, if you can post some more detail about what problem you are actually getting, we can try and assist you further.