I think dual wield would be the best complementary 2nd spec for pvp. For a stamina based, spammable, high damage ability I would recommend using hidden blade or flying blade from the dual wield tree. It is instant, cheap, ranged and does a fair amount of damage. You should watch this video of a vr10 nightblade in pvp, start watching from minute 11:00 and you can watch his scrolling combat text to see the damage of flying blade and how often he uses it.
You should note that in patch 0.180 they increased the stamina cost of flying blade so right now it is somewhat more expensive.