Bulldog-

Welcome to the forum.

I don't know of any shop in northern California that stocks double rifles... I can think of a couple shops that may from time to time have a couple, but not so much shops that stock double rifles per se.

There's a shop up in Perrin, but the guy there typically has whetever he owns in double rifle stock out at gun shows out in Nevada at any given time, there's a guy out in San Mateo county who evidently may from time to time have a double rifle or two on hand, also out in Walnut Creek there's a shop that (again from time to time...) might once in a while have a double rifle.

Something to consider is that these things are expensive and the gun buying crowd out here is pretty thin on appreciation of the double rifle.
What's your budget range? What's the asking price on that two barrel set you speak of?
If you're considering getting into a new rifle, Searcy down in San Diego county or the Rigby Rifle Company down in Paso Robles are the only games in town, and it's a bit of a drive (although worth it) to go in for an appointment with one of those rifle makers.

Do you already have any experience shooting such big bore guns as .577 nitro? If you end up considering Searcy, there's a chance he'd let you have a go with one of his .577 rifles there at his shop before you were to commit to such a machine.

I'm near you here in Oakland, by the way.

--Tinker