I will start off the post by posting the exact words of multiboxing rules for nostalrius as stated by 'Viper.'
Multiboxing refers to playing multiple characters simultaneously. This can either be achieved by using multiple machines to run the Game or by running multiple instances of the Game. The following rules are not meant to be exhaustive, and using multiple characters to level, farm reputation, run dungeons, do raids and/or engage in PVP is considered Multiboxing. Each case of Multiboxing will be judged individually and thoroughly. There are a few specific situations where you may have two characters in-game simultaneously, *only while both characters are in a major city*:
Transferring items between accounts;
Using professions, as long as no farming or material gathering is involved;
Crafting, enchanting, banking and/or for trading purposes that does not include the Auction House.
Portalling a second account only to and from major cities.
Playing simultaneously on both PvP and PvE realms. *Not restricted to major cities*
Now, the topic i want to bring up is having another account with a 60 paladin on it which would be used to be Mind Controlled by a priest, to give your main character (horde player) paladin buffs. This does not go against any of 'multiple characters to level, farm reputation, run dungeons, do raids and/or engage in PVP is considered Multiboxing.' Moreover, this mechanic WAS allowed during Vanilla times on Blizzard servers.
So, can a horde member have a 60 paladin on another account, so that it may be mind controlled to buff his horde character? It could even be in neutral city such as booty bay or such if need be (although that shouldnt matter since alliance and horde cannot open trade screen, group for pvp, dungeons, etc for any of the rules previously stated).