Gunman . That's me has been involved in the building /rebuilding of double guns for 45 year I feel I am qualified to make an informed comment as to what is correct and what is fantasy written by people who have never worked on or had any experience of gun making .
Shotgun barrels on a side by side are made so the tubes at the muzzle touch and are in parallel to the centres at the breech in a horizontal plain .
To make a double side by side that shoots over lapping patterns at 40 yard you will need to have the muzzle's spaced about 3/8" with a rib that stand 1/2" high of the tubes , work it out mathematically if you want , making a shot gun look like a double rifle .
I am sorry if this goes against what you would like to believe but as Granny said the truth often stings.