I’am currently starting a new epic quest with a friend in hardcore mode. He is building an elemental warrior tanking machineand me a classic melee warder playing primal strike melee.
We were building our devotions, and then a question came to our mind. If we both take the viper constellation, does the reducing elem res procs twice?
Anybody knows? Thanks you anyway 
No, it wouldn’t stack with itself.
This was posted on the forums here too, forget where. Not sure if its 100% accurate, but if so it should clear up how various resistance reductions work.
That has nothing to do with his question, but it is accurate.
Its absolutely relevant since it makes it clear what stacks and what doesn’t. It says that “% Reduced target’s Elemental Resistances” cannot stack, and that you pick the highest. If he’s looking to work multiple sources of resistance reduction for multiplayer, it could be relevant and helpful to provide this info.
Sure, you answered his question already with a simple “no”, but to say that my reply had nothing to do with his question is wrong.
Actually, his question was whether the same skill/debuff would stack with itself; which is what Ceno answered to. Your link is relevant to resist reductions as a whole, but not to the question of whether the same debuff would stack with itself.
“Cannot stack, pick highest” does imply whether or not Viper could stack with itself, assuming that the cheat-sheet is thorough and accurate. Please read.