What he's saying is that instead of having both point to the exact same place, have the least common redirect to the most common. This ensures that you use the most desired (normally the one that needs more legal protection) is the one that people are seeing.
But, DH can easily have both sites go to the same files, but it gets a bit hairy when you have an application installed. Most of the applications only know about a single domain so you find that the redirection trick can save a lot of trouble.
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