All I have semi experience with is Synergy in desktop modus at work. And not while multiboxing.

I heard synergy bugs a bit with the drag & drop in WoW though, not entirely sure the specifics of the bugs. And heard there are other ways of accomplishing that.

Alternatively, if it is just for switching, you can just get a KM switch for your mouse. But it does require a bit more input from you, whereas with synergy you can just move your mouse to other screens, with a km switch you have to.. well.. km switch, and then your mouse is on the other screen.