It's probably a redundant question, but I had to ask to confirm.
According to the Special Stance weapon skill, the debuff success rate is 30% cap together with other weapons. So that obviously means that you cannot put 2 
Cosmic Harp SPEC in order to achieve 40% in debuff success rate, correct? You'll only get 30% debuff success rate that way.
Now, there's a mention about other weapons, but it will have to be exact if it concerns and applies to all (same/other/all together) weapons. I cannot determine what those other weapons would be.
Example:
Weapon-wise, I can assume that I can use multiple (maximum limit is 4) 
Sharom Bow (I would mention 
Turpin Spear, but there is no Notes table validation on allowed stacking) to achieve that 30%. However, I am not sure if I can still stack up more than 4 to achieve 40.0% ~ 47.5% in debuff success rate, weapon-wise. If 30% is the limit, then 30% in the weapons gird it is, no more, no less. And that's omitting the characters and summons in skills, support skills, calls, and auras. The current hypothetical result would be affecting any special and dark class/characters/summons, particularly if they happen to be dark element and special style.
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I am itching to test out a least of 1 
Mithra and 1 Cosmic Harp SPEC. You can reach up to 40% ~ 55%? debuff success rate for special style characters. I also feel that there's a challenging (or even manly) logic puzzle that needs solving when pitted against same element foes in a debuff match.
