Multi Boxing addons are the only way. But.. You must own both accounts to be within the ToS and/or EULA.
At the moment, no one has said specifically that Multi Boxing is against the ToS or EULA. It can fall into a gray or black area from everything I've ever read depending on the way you multibox though.
I would take a look at the primary website for a better insight and maybe ideas though.
The only thing an addon is capable of doing independent of the local user's input is send messages through chat or perform emotes, and what I mean by "local user" is the player using the computer the addon is installed on.