Simple question on Sigils......
When you drop/apply the second sigil, a window will appear to allow you to choose which slot you wish to apply the sigil to.
Only on 2 handed weapons. Then it is as Inculpatus describes
You can now put two sigils on two handed weapons.
Most sigils can work in co-existance except for a few exeptions.
The ones that stack. Like bloodlust for example. You can have only one of those. Putting a stacking sigil on one hand and swap to the other hand with a different stacking sigil doesn’t work either as far as I know (Correct me on this)
AND you cannot use two ofthe same sigils in the same or different tiers. For example a 7% damage and a 4% damage does not give you 11% but 7% if i am correct.
What you can do is put a sigil of force on a sword, and have a sigil that gives you a 60% chance of might on a crit on a warhorn. You will receive both the damage and a chance on the might if you crit with your sword.
Putting a sigil that give an AoE Flame and an sigil of Air which can give a lightning strike on one two-handed weapon does also work, you might even get both effects on the same hit.
I am sure about everything except for the stacking sigils on weapon swap.
Hope I helped.