There is a book out there on making Double Rifles from Shotguns. I can't remember the author or the title but I'm sure someone can.

It is quite doable by a competent hand, the only tough part being the regulation which is quite easy to understand and quite difficult to actually do.

I have a 303 that has only two under lugs and it seems to work just fine.

Just to be on the safe side I would look for an action that has three fastners. I think that if you could find one in a 16 ga it would be nearly perfect. You can monoblock the barrels or start fresh with chopper lumps. You can buy the chopper lumps from Galazan and If you want a sweet English action you can get an out of proof shotgun action, with all of the hardware, at auction in England.

Add all of that and allot of expertise and creativity and you are off.