Does Item Increase stack?

[Raven]

Replaying the game, currently combining accessories, and as I'm examining my options, I'm presented with the choice of making a second Avarice Jewel and therefore having 2 accessories with the Item Increase skill and possibly doubling my chances for loot. Tempting, but I feel like it doesn't stack, which makes sense and therefore makes the combination worthless. But I'd like some experts' opinions before making a choice.

[Raven]

I just ran 3 trials, and I have a sneaking suspicion I misunderstood the skill. There's a possibility it only affects the drop rate if the character with the skill is the one that dealt the last blow, whereas my first assumption was that it affected the whole party.

Granted, it's categorized as a skill that indeed affects the whole party, but given my trials, it might only take effect when the equipped character deals the last hit. Iunno, I'd like more thoughts.