Paul, it's not clear from your posts whether the gun in question is a 12 or a 20. If it's a 12, you could buy a relatively inexpensive (relatively, not absolutely) BSS 12 with the selective trigger, and have your gunsmith swap the frames between the guns. The barrels are interchangeable between the NST and SST guns (with some filing/fitting usually required). You get to keep the "fitted" gun and you could then sell the newer gun with the older frame. You would probably take a hit on resale because of mismatched serial numbers, but probably less net cost than fitting double triggers. If the serial number mismatch bothers you, have your smith swap the trigger assemblies between the two guns (and disclose this when you sell the newer gun). It's not the DT swap you asked about, but it does give you a selective trigger. If your gun is a 20, same principles apply, but cost of entry is higher.
Not too many parts are actually interchangeable between the BSS and the pure Miroku or Miroku/Daly boxlocks (despite sellers' claims that the Miroku is just like a BSS, but cheaper), but there is a gunsmith in AZ who advertises on GunBroker. He has a slew of Miroku SxS parts and would know if the DT swap is doable with available parts. Do a GB search for Miroku and his partial list of parts should come up. If you can't find him, let me know here and I should be able to find his contact info for you.
