You might want to consider building a time of day schedule though your voice mail system, so that when the calls come in on the main number, it goes straight to voice mail, through a time of day schedule, then sends it back to the main phone (will have to have the main number changed) during the day, or to an MCR key that appears on the main phone and the secondary phone during night hours. After this you can add the MCR on multiple phones if you wish without having to mess with any other parameters, other than adding the MCR key.
The main factor to think about in this scenario though, is how amny lines you have to and from your voice mail and how many calls you get. If you have a lot of calls, you could end up blocking voice mail access, which may not be a good thing, but a few should not cause any problems.