It doesn't stack anymore.
Now, I know this is the way it is supposed to be and I was the first in line to think this because it is an "Equipment bonus" like every other item BUT +2 stacking DC is going to become useless when we have up to +5 DC.
The way it was working (the not WAI) was actually pretty decent. It forced us to fit in our equipment another effect while giving us a not so bad bonus to our DCs which doesn't hurt us.
Chance the Equipment bonus to be a stacking bonus, like Exceptional Combat Mastery for melees.