Daryl:
In my experience very few mid-to-modern SxS shotguns have a choke anywhere near suitable for rifling to make a paradox. Most are way too short in the 'parallel-constricted' bit. I've been looking!

Some of the latest steel-shot chokes may be better, but not easy finding one with fixed chokes the same in both barrels, or with barrels that aren't chrome-lined.

Good in theory, but fails in practice.

BTW, the Rem-choke rifled chokes are probably still available, stainless steel with concave grooves, but in an 870 single they don't work as well as they should because the start of the leade is recessed about 10 thou, so in effect the rifling is a bit recessed too, so not as aggressive as it should be. I bought a pair of these years ago, as a back-up option for my Paradox aspirations, but haven't been comfortable fitting them to anything yet.
Cation and Hastings (and others?) also make rifled chokes, but they protrude and would be unsuitable for a SxS double.