I have two sigils one in each dagger the first being minor sigil of bloodlust which is 5 power and stacks once per kill 25 times, the second is major sigil of luck that stacks 0.4 magic find once per kill 25 times.
The issue is only the dagger in the main hand is the one that actually gains stacks.
I’m not 100% sure this is a bug and I could understand two versions of the same sigil not stacking but it seams kind of broken that two different sigils that only share the stacking mechanic don’t work with each other.
If it’s not a bug then there should be a heads up warning in game to inform players that only one of this type of sigil can be active at any one time.