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.