NitroX, No gun maker here, but I too studied the double rifle characteristics before buying mine. Bought some books and did my home work. The two most helpful books were as follows: Building Double Rifles on Shot Gun Actions by W Ellis Brown, gunmaker in Colorado. This book directly addresses your questions. and The Double Rifle Primer by Cal Pappas -well known African hunter, gun collector, and author in Alaska. Very informative and a good read.
It appears that a couple of the previous posters on this thread have also done their homework.
-------------------- CRS,NRA Benefactor Member, TSRA, DRSS, DWWC, Whittington Center
Android Ballistics App at http://www.xplat.net/