You'll really need to get to grips with using keys if you're going to multibox.

The transition from clicking will be much easier if you get a mod such as Bongos that presents a MUCH more user-friendly interface to set the bindings up for you, and can also be configured to display a reminder as to which key you've actually bound to each action, right on the icon itself.

It won't take long to get used to. Honest 8)