I don't know, but Ellis Brown does.
http://www.rayrilingarmsbooks.com/cg...b455.cgi/10382
BUILDING DOUBLE RIFLES ON SHOTGUN ACTIONS.
by Brown, W. Ellis.
Publisher Information:
Bunduki Publishing Ft. Collins, CO 2001
This book is written to take the gunsmith or advanced hobbyist step by step through the process of building a double rifle, using the action of a side-by-side shotgun. Chapters include evaluating actions and cartridges; building monoblocks; ribs; bushing firing pins; and proof testing. The author explains what regulation is and isn't, and offers his proven method of regulating double rifles so that both barrels shoot to the same point of impact. Brown details each step of the entire process, to end with a functional, well regulated double rifle. 187 pages, including index and black & white photographs.
You and your guy might look around on Nitroexpress.com in the gunbuilding forum.
http://forums.nitroexpress.com/postl...Board=dblbuild