As far as I know the only way to violate the TOS in regards to multi-boxing is by using 3rd party software and bot'ing. Which makes me question using Multiplicity or Synergy... has anyone had issues with Blizz in regards to either of those titles?

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I noticed in one of the hardware/software threads a similar discussion, and it appears its legit for the time being
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As far as running multiple instances of WoW, I've been using 2+ on the same PC since launch and have never had an issue with players or GMs. Its not by any means effective though, other than for casual support, item exchange, crafting, etc.