If you set your phone type as "Home" in your Google Voice settings instead of "Mobile", you can attach it to another Google Voice number. I currently do this with my front door callbox, and it works like a charm. I think the limit may be two Google Voice numbers to "Home" real-world phone number, though.

One caveat: Text messages will only be sent to phones listed as "Mobile". But I just get my text messages via the Google Voice app anyway, so this is actually a benefit to me (no duplicated texts).