So there are 2 types - Weapon specific perks
Others
If you have 2 pieces with, say, Keen Tondo, you will benefit from the better one (9.9% vs 9.8% crit)
If you have 2 pieces with KEEN (shield-sword) - you only benefit from the better one
If you stack ANY gear perks - they stack ADDITIVELY (all wards, all refreshing perks, all fortifying perks)
Keep in mind, your fortification cap is 50%, as well as Rend. I feel like, weaken is the same, capped at 50%
Leeching-Lifestealing stack
Keen-Keen awareness stack
Resilient stacks
Luck perk might not stack on both your shield and sword, however, no idea, this is really hard to test, whether shield luck even works.
Alike perks do not seem to stack on your sword shield (Rogue - Vicious), anyways, ARE pretty useless as there are better dps weapons
You can always find out, whether you should look for a specific perk-combo-piece of gear IF you can chose these perks in a TP filter. If you can’t and it says “perks are exclusive…” - you cant have such perk combo proc on any random drop, MAYBE the specific combo might be true to life on a NAMED weapon, go to nwdb for that info.
Yes, additively
If you have your sword and shield UP.
I stack my refreshing evasion on every piece of gear with bow and my CDs come back up in half the time. 2 light shots, dodge
With refreshing move on a bow and internal CDR 30sec cooldowns are more like 12 sec cooldowns
But 590+ RefEvasion+Resilient+Dex cost me a fkn fortune
ohhhh I rolled 600 helmet yesterday and thought I have to sell it because it has the same Refreshing ward as my shield… now I see thats ideal for my tanking haha
If you are not sure whether perks stack, I suggest this - buy the cheapes possible FULL set with the perk and try to activate it wearing full set - you will get a result.
After, use only ONE piece with the given perk and compare the two results. That is how you know, when it stacks or not