My practice is to use use the to: line for those that I expect action from and the cc: line for those that need to be aware that requests are being made.
In the the body of the message, all requests for action are preceded by the name of the person that should be doing that action.
The other things that I've taken to doing is modifying the subject header to indicate the actual subject, so if someone sends a message with the header that is not specific enough, I'll modify it.
If some sends a message about two different things, if it's complicated enough and both things need to be tracked separately, I'll send two different replies, one for each item or issue.
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