Strictly speaking you can dualbox without it. Works for me anyway. Synergy, however, will be useless if you don't get it as the point of getting such a program isn't to be able to move your mouse from screen to screen but to be able to broadcast your keyboard.
I couldn't get it to work and ended up with 1 keyboard in each hand. Well, at least this way I got my movement keys enabled on the alt in tight situations.