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Multiboxing Frequently Asked Questions - READ THIS BEFORE POSTING.
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Which bits didn't you understand?
Just use a keyboard multiplexor program, then press, e.g. '1' and all copies of WoW see '1' and cast that spell. Easy.

For buffs like lightning shield, install the SmartBuff addon for all characters, assign SmartBuff to a key, then just bash that key to buff everyone with everything.