Boxing multiple classes is done all the time, the trick is figuring out your bindings, which will largely depend on how you play. For instance, if you button mash the button 2, you won't want an insta cast mana consumer attached to that button on your priest. You will however want keys to control doth toons close on hand, so i reccomend against the num pad unless you don't plan on using your mouse much.

G Keys on the G11 and G15 are remapped to keystroke combinations eg Ctrl+F1 on G1, this is good because it works flawlessly with keyclone.

And yes the price of keyclone is now $20, but since you plan on using a single computer thats it, and you don't have to pay more for future updates or support.