First welcome to the club. I, like I think most of us here, also love doubles. I applaud your desires and wish you the best. Not having the skills you do, it would be impossible for me to attempt; but Ive designed several DRs in my mind.

There are a few sources that you might want to check. One is a book "Building Double Rifle on Shotgun Actions". What you are talking about HAS been done. and done well. Its also been attempted with poor or incomplete results. If I was going to do it I would not piecemeal it. Either do it or not, do not plan on buying a few bits at a time. If you dive in youll be more inclined to complete it and less willing to abandon the project along the way.

The cartridge you chose seems like a good one, it might not be historic as a DR cartridge; but it should work for what youre talking about. Ive often wondered why the big manufacturers havent made a double rifled 12 or 20 gauge.
Regulation is said to be the big issue. I cant help beyond moral support.

Best