The game guide is not entirely clear. It says debuffs do not stack unless they “directly modify specific stats or resistances”, in which case they do. And what qualifies in this case as a “stat”? Would one that reduces the same stat by % and one by flat amount stack? Is it just that the effects need to have different names…(the full description sort of alludes to this)? Just clarification in general.
From my Blademaster build guide thread:
Regarding the special exception:
Buffs, Debuffs and Damage over Time
If you apply a % reduction to a monster’s damage by using a skill and then apply the same effect with a weapon attack, only the strongest debuff will take effect. The effects do not stack. In the case that a strong effect has a shorter duration than a weaker effect, the weaker effect will become active once the stronger effect expires. A special exception to this rule are debuff effects which directly modify specific stats or resistances, such as -% Vitality resistance. If two differently named debuffs (ex. Curse of Frailty and Devouring Swarm) both reduce a target’s specific Resistance by -%, the effect will stack as they are two unique debuff effects altering the same attributes.
If there was another skill like Vulnerability, then the -DA effects of both skills would stack, too. But if I am not mistaken, we currently do not have such a skill … or maybe as an item skill? Anyway the Game Guide explains that scenario with resitances, which is more common.