Greg,
If you have a specific gun or two in mind, knowing what make they are and whether or not the triggerplates have two slots or one would be helpful.
For example; the Browning BSS guns have two slots in the triggerplate already set up for the double triggers. Converting a BSS to a double trigger arrangement would be a matter of making the triggers and gutting the single trigger mechanism, then fitting the double triggers. Still no cake walk, but not too bad.
On the otherhand, my old NID single trigger has the triggerplate with a single wide slot that would have to be welded shut and remachined, then the sears, which are curved upward would have to be straightened and new triggers fitted. It'd be easier to just steal parts from another DT NID. But the above illustrates the type of work if parts can't be found and have to be reworked or made.
