Fred,
Despite what some may tell you, it is possible to build a very good double rifle using a shotgun frame.

The biggest pitfall is in regard to the man who is doing the work.
Many gunsmiths IMO should not be allowed to even work on double rifles, let alone try to build one from scratch.
You really need to see and shoot an example of their previous work (building doubles) before trusting them to be able to give you good value for your money.
Thesedays it is not really economical to build double rifles as there are lots of over the counter new doubles available at fair prices from the likes of Merkel, Chapuis and others.
Best of luck with it if you decide to go ahead.