you cant use them both at the same time, basically you have to use them one after the other, so unless youre trying to pad numbers on something like beastlord.. you shouldnt use them together.
but, if you did want to pad numbers and dont care about boss dps, you use each "on use" when adds spawn..
then again, if youre having to ask this question, you probably should focus on other things besides padding numbers on useless things that wont help you get a kill.
tldr
use the vial and skull of war for now.
