Its not in the Terms of Service, its not in a sticky, its not in any of the FAQ's, its not in any announcements ingame or otherwise.
Even the best intentioned of players who wishes only to do the right thing cannot follow rules they are unaware of. I multi-client to play in more than one battle room (I saw other people doing it), and accidently joined the same semi-pro room twice rather than the two different ones. I ended up fighting myself and while i thought this was merely humourous, i was informed that it was not permitted. I closed one of my clients immediately when told this (No win was recorded). I wish only to obey the rules set down by the community/moderators/developers. If this is a bannable offense, it would be nice if players are made aware of it beforehand. I had no inkling it would be illegal, since i assume (as a game developer myself) that these sorts of things will be forbidden by server-side restrictions if the developers wish to forbid them.
I say this in the hope i shant be banned for this confession (again, no wins against myself were recorded) and merely in the interests of preventing future violations of this rule by those ignorant of it.