[Shodo]

If the situation you described is indeed accurate (keep in mind here that you only know the details indirectly, and the person(s) controlling the suspended accounts may simply have lied to you for one reason or another), it is not considered two-boxing and you should have nothing to fear. That being said, keeping an eye out for suspicious behavior is part of a GM's job, and the situation you have described is the very definition of "suspicious behavior" as far as two-boxing is concerned. So, if it was a mistake on the GM's part, it's an understandable one (although typically they will have the participants perform simultaneous actions or something as a simple test).

If you're worried about receiving a similar punishment, simply avoid those types of suspicious situations. Having one toon essentially go afk, only sticking around to buff and vendor, while the other levels will always toss up a red flag as this is very common two-boxing behavior.