The SNS triggers I have seen and worked on inclu the above mentioned were all a 'contraption' of stamped metal parts and coil springs.
The metal parts were punch press fabbed & then bent in mfg'r were needed to be formed in those shapes. The main spring is an extension type coil spring, not very heavy.

Simple straight pins held things together. Nothing too precise actually.

The entire mechanism works off of the right hand hammer.
When cocked,,that hammer pushes a Slide Bar to the rear that is centered over the Single Trigger ,,and underneath both sear arm extensions...
That 'slide' has an extension off of it that bears against the upper portion of the back side of the RH hammer and is spring loaded lightly towards the muzzle with that extension coil spring pulling it in that direction tied to frame.

The slide is a simple flat piece of steel stamped out and properly bent to position itself.
At the rear of it are open slots on each edge. One on each side.

When the hammers are cocked, the right hammer has contol of the slide and has pushed it to the rear.
At that point the Open Slot on the left side of slide is positioned under the left sear arm extension.
The opening on the right side of the slide is to the rear of being under the right sear arm..The solid surface of the slide itself is underneath the right sear arm extension.

Pulling the trigger lifts the Slide Bar up.
Being no opening on the Bar under the RH sear,,the bar lifts and fires the RH sear/barrel.
At the same time,,the lefthand side of the Slide BAr is being lifted by the trigger motion,,but,,there is blank/opening in that LH side of the bar.
So the Bar moves upward and past the left sear arm extension w/o touching the sear extension
and the LH sear/bbl does not fire.

Releasing the trigger after the RH bbl is fired lowers the Slide Bar. Now that the RH hammer has pivoted forward,,the slidebar is no longer positioned the way it was with it's extended arm bearing against the upper portion of the cocked hammer.
The extension coil spring powering the Slide Bar pulls it forward a bit to reposition it and ready for the next shot.

In the slide bar repositioning itself forward, now that open slot on the left side of the rear of the bar has moved forward and a solid surface of the bar is now positioned underneath the LH sear arm extension.
As well the solid surface of the slide bar that allowed the RH sear to be fired has moved forward and now the empty cutout slot on the RH side is under the fired sear on the RH side.

The second pull of the trigger lifts the entire Slide Bar again just like the first pull did,,but this time, the LH sear arm is disengaged as the Slide Bar solid surface pushes on the sear arm.
The Left bbl now fires. Your second shot w/the SNS trigger.

The open cut on the RH side misses the already fired RH sear arm, if it's at all in contact, which can add extra trigger pull weight on the trigger. It all depends on if the RH sear kicks out of the way or not upon release.

Opening the gun cocks the hammer(s). The right hammer pivots back and cocks and at the same time pushes the Slide Bar back again and holds it. There the rear slots are in position the fire the RH bbl with the first pull and then the LH bbl with the second pull of the SNS trigger.

This is what I remember them as,,I may have a couple details out of line.

Safety blocks the trigger only, just as most all SxS trigger mechanisms.

They don't look like much, but they work well. Most adjustments are by bending and 'tweaking'. Smoothing up edges from the orig punch press fabrication process helps alot.
If I remember right that Slide Bar was at least lightly case hardened on one or 2 that I worked on.
I can see where these mechanisms could be factory made and then installed without much technical expertise, fitting and problems.