You can set up a hunt group with both phones in a extension list then use a CRA as the recall destination to transfer the call to a specific mail box.
That being said, DEE or “twinning” would be your best option. That is exactly the situation it is designed for. The two best reasons to use DEE is that the user will keep his existing extension (instead of having to use the hunt group ext), and, as long as your software release is up to date, you can get Message Waiting Indications on both sets! (I believe this was introduced in 6.0, but not sure).
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