I do get whispers when I'm leveling low level toons dualboxing because I make them identical 'twins' for the sake of easiness. By being the same race and class (and going all out to have identical appearances) my toons are recognized as multiboxers all of the time.

On the other hand, my Belf Priest and my Troll Hunter have never once received a whisper from a stranger asking if they dual-box. So if you really don't want to be harassed but want the fun and challenge of multiboxing, I'd go with different races, different classes and/or different appearances.

(Just in case someone is using 'search' on the forums and actually looking for info on how to go 'under-cover')