If a PC has defiant bonus on both the armor and shield, does the DR stack so they'd have DR/4- versus the creature type?
DR never stacks. Only the single best source applies to any attack.
As well, bonus' from the same source never stack as well. You cannot double or triple up on the exact same magical enchantment and have them stack.
DR I can understand but the enhancement bonus I would question.
The wording says "the item's enhancement bonus to AC is +2 better than its actual bonus". It does not name the bonus but says it is the same that is currently on the item. So you would have a shield bonus and an armor bonus.
The bonus is an ENHANCEMENT bonus to aC and AC only. An enhancement bonus increases the value of the item enhanced.
So a +1 heavy shield with this would be AC 5 and that would all be Shield AC.
A +1 Breastplate with this would be AC 9 and that would all be Armor AC.
The enhancement magic icreases the existing items AC and the increased AC stays the same type, IE: Armor or Shield.
So the Enhanced Armor and Enhanced Shield AC's would stack, just like Armor AC and Shield AC always do.
But DR never stacks, period (unless the sources of the DR specifically and explicitly state an exeption). You always choose the best one for the attack your defending against if you have more than one DR source.