Pugwash is giving you information that is entirely correct.

Further, if you desire to build a proper double rifle in a large caliber, his suggestion to buy a sidelock back action from a gunmaker is straight on.

Having built some double rifles myself, I know what I would do to attempt to find such an DR action from a gunmaker. Arrieta in Spain ceased gun and rifle production; and they made some of the best Spanish double rifles. There must be some double rifle actions in their old shop. John Boyd of Quality Arms Houston, Texas has been a dealer for Arrieta for about 40 years and knows the owners of Arrieta well. What I would do if I were in your place is to telephone John Boyd and ask him to get in touch with his Arrieta friends in Spain and ask them if they have what you need and are willing to sell it and ship it to John Boyd. They likely even have barrel sets. Of course, John Boyd should be paid a fee for doing this.