Originally Posted by
nighties
Though I'm not entirely certain which hand the sigil has to be on, I do know that 2 sigils that grants x boon on kill does not stack with each-other. This includes the mainhand + offhand / alternate weapon set. So you can't use both sigil of bloodlust and sigil of corruption. This goes the same for sigils that grants boons on weapon switches.
This was intended so classes with multiple weaponsets (every class but ele/engi) won't have an unfair advantage on those who don't.