I just started dual-boxing on two computers recently, and I just wanted to share the one really helpful thing I've learned to do. I hope this isn't obvious and hasn't been suggested like 100 times before. The scroll wheels on mice can be keybound just like any other button can. Every time you scroll a mouse wheel down and you hear a little "click" it's an individual button press. So scrolling down repeatedly basically means you're using a macro/attack as fast as you possibly can, while also being incredibly easy. I have it set up so scroll up = target next enemy on main, follow on alt, and scroll down to be the attack macro for both. You don't even have to touch the keyboard, you can control both characters with just the mice - with a few keyboard commands for emergencies.
Hope this helps!
- a dual boxing newb.