How do special effects work with dual backstab? Do they come into place before of after the second stab? I'm thinking of spine hit specifically since the second hit would stir them from being spinebroken and begin combat?

Or do both backstabs have a chance of extra success or extra failure? Could you do an extra effective backstab and then sprain your wrist while auto-attempting the second one? In which case taking strongarm backstab edge would seem unwise if you have the skill.

It's there to add a little oomph to backstab against foes with heavy DR.

I tried a duerg with strongarm & dual backstab.

It was a while ago, but I seem to recall if you get off the second backstab, you have a chance to land a special effect on it.
(I imagine it would bring them out of spinebreaker if the first one did it though)

Only problem with dual backstab, it does not fire if the guy goes below a certain hp% after the first backstab, (I think it has the same parameter as if you can backstab them normally- so basically at covered) which most people almost always will. Imms could probably confirm this a bit a better though.

I had a vision of strongarm + dual backstab taking most people to gushing or wirthing because of the 2nd hit, but more often than not, you're just getting 1 hit because they get too hurt by it. As a caveat, I had no idea about those limitations when I created him because I didn't bother to look things up.

I didn't think the thief points for dual backstab were worth taking it. Then again, I know squat about playing thieves, but on a duerg it seemed pretty much like wasted points. The only things I ever insta killed were conjurers. Backstab familiar and insta-kill it and the familiar's death then kills the conjurer.