Q:
offhand sigil
If you’re trying to use two sigils that have a stacking property (such as Bloodlust and Corruption) then you should know that only one of them will work at a time. Whichever stack you start with, you can only increase that stack, making the other sigil useless. All other sigil combinations should work normally. For example, I have a Sigil of Bloodlust on my main hand and a Sigil of Speed on the off-hand, and the stack works and I get a swiftness buff on kills.
Blackgate
1) If you have x2 Kill Stack sigils, then only the Main Hand slot will work. For your Off-Hand, you’ll need something with a static effect (like +% Damage or +% Duration) or a Crit Proc sigil.
2) If you have x2 Crit Proc sigils that are off different types, then only the Main Hand slot will work. For more information, see Response #1 above.
3) If you have x2 Crit Proc sigils of the exact same type, then it just increases you Proc Rate and doesn’t cause a 2nd individual proc to fire off seperately.