I got in trouble for doing that while camping a rare spawn. I was told to either get them all in one spot under my control or have the other toons sitting elsewhere passive. And when I was attacking two places at the same time once I was warned about that too.
So yea all your toons must be under your control at the same time. As in you cannot have 20 hunters sitting afk in a field killing everything while you are on another toon running a dungeon. People did that and so that is why Blizzard removed the auto attack from pets, minions and demons. Multiboxers could simply sit afk in a spot and come back to it and make a fortune. There were also single cases of players doing this as well. It is against the TOA to allow your toon to do actions that you are not in control. How fare would it be for you or me or anyone if a multiboxer had a raid group full of toons on a PvP server and they were all hunters and warlocks with all of their pets set to attack… this was before it was turned into assist. Partly that reason and also partly because huntards kept accidently pulling raid bosses.
So I was told that you are responsible for all actions done on your account. If you do not believe me then try going afk with multibox hunters who are auto attacking everything. You will be banned. Your toons must be under your control at the same time and yes I did get into trouble in the past for splitting up my group and doing separate actions on them. That is why now when I do multibox I will keep my inactive toons in a city. if you do not believe me that a ban will not happen if you sit your hunters out afk and let them kill everything while you are logged out in another toon doing something else, think again. Blizzard tried to stop this exploit by removing auto attack, but it still happens from time to time.
The moment any of your toons starts doing reparative actions that you do not control, that is considered an exploit. People are somehow able to cause their warlocks and hunter pets to auto attack even with it turned to assist. They would exploit the phasing, all would split up and then you would have a dozen or more toons sitting in the exact same spot, killing the exact same mobs, but each could have an unlimited amount as they were not sharing kills… phasing. That is why Blizzard removed auto attack on our pets. And even then they were able to create macros to auto attack. I am fairly certain that THAT is the man reason why I was told that your toons must be in your control at all times.