The bonus effects of the items that increase defense and offense don't stack up.
When you apply another item of the same type it replaces the one which you had activated previously.
For instance. You purchase an item +50% Offense and Defense Enhancer and it is automatically activated. If you try to activate e.g. +20% Offense Enhancer or +20% Defense Enhancer, you receive the following warning message
In case you press “Yes” your +50% Offense and Defense Enhancer will be replaced. Moreover, it will unavailable for the repeated activation.
Read warning messages - don’t shoot yourself in the foot. Forewarned is forearmed.