Instructions
Equip the W-Item materia on the desired character. This will enable the "W-Item" battle command.
Enter a battle and await the turn of the character equipped with the W-Item materia.
Select "W-Item" from the battle menu and press the circle button. You will see a list of the items in your inventory. Select an item from the list with the circle button, then select which character you would like to use it on by pressing the circle button again.
Press the circle button once more to select the second item you will use. At this point, you can either use the item or perform the item duplication trick. If you wish to duplicate the item, cancel the action with the X button. This will add one more of that item to your inventory. Alternately pressing the circle and X buttons will continue to add items until you have a full stock of 99.