It isn't against the rules to have characters of opposing factions on the same server when you have multiple WoW licenses under the same BNet account, which is why JohnGabriel's link to the Blizzard forum specifically states "per WoW account." However, as my link points out, cross-faction collusion is not allowed, and, as a multiboxer, you're simply bypassing the barrier put in place in order to stop this behavior.