Okay. Are you looking for an O/U, double gun, like the Beretta, or a side by side? Or a swivel Breech?
The side by side DB rifles are the most difficult to build and regulate, even with a monobloc. It has everything to do with the barrel harmonics of the two barrels, and how they have to be attached together in a side by side. In an O/U gun, you don't often see the two barrels soldered together the entire length, which allows the two barrels to vibrate somewhat independently of each other. If octagon shaped barrels are used in either, you get more rigidity, which lessens but does not eliminate the barrel vibrations. Because of the added weight of a two barreled gun, most double rifles, breechloading or MLers, tend to be made in smaller calibers( .45 and smaller) and the barrels tend to be shorter than most single barreled custom guns.
By all means, read the book, and consult the tables. They will be of great help when you are trying to regulate the barrels.
Personally, I have always thought owning an O/U rifle, with a 20 gauge smooth bore on top, and a .36 caliber barrel on the bottom would be a fine combination gun, that would not be too heavy, and would balance reasonably well.