Multiboxing actually refers to using several computers (multi *box*) to play multiple accounts. I get your point, but it's void since it's not what I was referring to in any part of my post. I, personally, have used a dual-monitor setup and two clients, one on each screen. Fully within the EULA.
I could also have used a program like Synergy to share my mouse and keyboard to a second computer and screen without breaking the EULA (it works exactly like a 2+ monitor setup, but I don't have 3 paid accounts so it's not needed).
People macro'ing more than one ability to a key, be it with one, two or ten clients may be breaking the EULA but that's not a required part of multiboxing in many games out there, including AO.
It's also not breaking the EULA if you bind, for example 1 2 3... 9 0 to abilities on one client and, for example, alt + 1 2 3 .. 9 0 to abilities on a second client (or a second PC). You're still using one input per ability. Just make sure alt-1 to 0 aren't also mapped to stuff on the first PC. I wouldn't do it because I personally prefer the dual-monitor solution, but that's down to personal preference.