Here is a link to a kind of a weekend (well maybe two) modification of a douleneck guitar turned into a doubleneck guitar/ mandolin.
Since we make thousands of fingerboards here, I may put together a kit for this. Haven't done it yet but if there are at least 10 people that want to create one of these frankenstiens I will do it. The whole article is here:
http://www.allenguitar.com/dbl_nk_mando.htm