Welcome. You have stumbled upon in my opion the most experienced group in cyberspace regarding this passion. My own present project is a sidelock hammer gun converted to .30-30Win.! Not as talented or ambitious though I started with an existing shotgun action... well, most of one anyway. Maybe someone can provide the drawings you seek but if not reverse engineering is pretty straight forward if you can get your hands on even an ancient shotgun of the type desired. Even lacking that the mechanics of a double gun are not all that complicated... until you try putting it together at least, and a rudimentary understanding of what's needed can be arrived at from even crude sketches and very different action styles. Unless you are a purist who seeks to exactly replicate a specific firearm I think you'll be able to work it out. Good luck and please keep us posted on your progress!