Though the tutorials will explain the intricacies of bonuses such as Stack, the way to get this trophy is actually quite simple.
In the first area, be sure to open all chests you find.
Two of them will have gear.
(Ancient Earrings would likely be the first one.)
Each of them will be the same as the gear your characters have except for an extra Spirit attribute, which happens to match an attribute you already have.
Which means equipping the gear would have this Spirit attribute "stack".
Equip the gear to earn this trophy.