here is how i remember testing %total damage:
i used warfare + dream (what's that called again?)
dream because the damage shown from distortion wave is always the same, so that's the damage you check using show damage option
first, to be more accurate, i unequipped all items
i hit dummy or any other enemy, and noted the damage (say 100.. i forgot the exact values)
then i equipped 1 neiths will and checked the damage (say the damage went from 100 to 120)
then i unequipped the 1st neiths will and equipped the 2nd (say the damage went from 100 to 122.. %total damage is not always the same)
then equip the 2 neiths will and again check damage
so if the stacking is additive, the additional damage should be 42 (20 and 22)
if it's multiplicative, it should be lower (25% from the 1st neiths will, then 25% of 25% from the 2nd)
if you test it yourself, you will see the additional damage doesn't get lower
i even equipped myrmidons pendant and it's still the same (it was as if it's additively)
finally, i checked this pet damage guide by Vio
https://www.kirmiziperfect.com/titan-quest-all-about-pet-damage-by-violos/if you scroll down to build example under practical example, he counted 3 ritualist jewelries as 36% physical damage (that's additively)
he even added 90% physical damage from strength of the pack as if it stacks additively
now, i don't know how he does it but when it comes to mechanics like these, Vio knows his stuff