in the callFWD model, callManager, or whatever PBX forwards the call with the original called number in the details. this number is used to revert back to a voiceMail box if the number forwarded too does not answer.
to get unity to send the forwarded call to the 'second' phones voiceMail box, you can use a call handler or just edit the original called subscribers unity configuration as follows:
on the 'greetings' page of subscriber configuration in unity, in the 'afterGreeting' section, you can send the caller to a subscriber or call handler. this will allow the caller to place a voice mail in the second phones VM box instead of the original called numbers VM box. (this is one way to do what you need)
or you can use the