Nothing illegal in dualboxing, but if a GM asks you don't know your fiancees' password and she just logged on and left. Also you got no idea of her account details (other than her name) or anything else she's not supposed to share.

Other than that Blizzard don't mind dualboxers, if they had anything at all against us they'd have listened to the whiners long ago and put it in the ToU or smthn.